Part of it is intentional part of it might not.
When you try to heal a fighter that is in close quarters with the zombies it's possible for the doc to get suppressed (like he's being fired at in a raid)
The weird thing is he will still heal.. kinda.. it's like the healing goes into escrow.. it's happening but does not make it to the fighter and will still take damage.
if the fighter is able to survive the fight and doc will go back to normal and the fighter will get a huge influx of health suddenly.
I imagine the suppression of the doc was intentional.. to keep you from just out healing damage.
now the lock out you mention may not be intentional.. see what happens is when you set to heal someone their action is canceled and no matter what they was doing besides looting they will stop right where they are till the doc comes and heals them.
it was probably written this way because A: you have stand still when getting medical attention IRL anyway.. and B: the animation would be really weird if 2 survivors tried to run and heal at the same time.
The bad thing here is as you've learned if a survivor is set to be "healed" the survivor can not be controlled, the doc overrules the actions.
This is not really a problem except when the doc is suppressed.. if you try to move the fighter away from the threat to let others take the heat for a moment it does not work because the fighter is "being healed" except they're not.. the healing is going into some sort of escrow while the doc is under suppression.
your only choice here is to move the doc away to beak the lock and then move the fighter, which can be deadly sometimes if the damage is stacking up quick.
So here's my advice.
1. put a helmet on the doc that -% suppression.. this will help a bit but not solve your suppression problems.
2. heal in short burst.. do not wait for your survivor to be at 50% health, heal up as often as you can.. smaller burst = less likely to be suppressed
3. put +heal gear on your doc.. if it has +health all the better, do not put combat gear on them unless it's all you got.
3b. put a pistol or smg in your doc's hand.. he can pitch in and with damage but remember first job is to heal.
4. get a 2nd tank.. I go with a 2 melee blunt, doc (pistol/smg), and 2 LMG/AR
I go with blunts because of the knock back, blades do more damage but have very little knock back.
Knock back is extremely important for managing a crowd it stuns the zombies for a moment.. so if you have 2 tanks with melee blunts they can keep a crowd pretty well stunned up unless it's really bad.. their job is crowd control more then damage.. your LMG / AR users will be behind them dealing most of the damage, the doc pitches in with damage but otherwise is mainly there to keep everyone breathing.
try to move as a group into each loot area.. if possible keep your tanks facing spawn points.. if it's not possible then position your doc in the middle, with range on either side of them and tanks in front of the range users
tank - range - doc - range - tank.
I never see more then 2 spawn points active at a wave.
With this sort of formation you really just have to manage the doc during waves.. heal up the tanks when they need healed
If you're in the "optimal" position that you have the spawn coming directly at you and engaging you from 1 side then both tanks will usually handle a crowd pretty well.
Melee users will run ahead a bit of your group to fight if you find one of your tanks needs healed pull them back and allow the healthy tank to maintain the crowd for a moment.. you have to be quick to heal them because as soon as they get to where they're going (back towards the group/doc) they will turn around and head back into the crowd.. so as soon as they've broken free of the crowd sick your doc on them.. they should heal up fast and then automatically head back into the fight.
Juggle tanks as needed to keep things healthy.
Now when I say tank I usually mean fighter, but not necessarily, tank is just someone who is designated to take damage.
namely this is fighters.. but I've also found my scavenger and engineer to make what I'd call a subtank, or tank assist.
Your tank should have melee and armor (Primarily melee resistance & health, +melee skill is nice but secondary)
Now my scav is pretty good at dodging so thats what makes him acceptable for front line combat.. I would not want him as my primary but he can keep a crowd under control for a short period of time while the main tank is healed.. same thing with my engineer, strap some decent armor on them and a good blunt and they can pitch in to keep the zombies under control... remember "knock back" is VERY important.
another thing about using non fighters for your 2nd tank.. try to position the main tank slightly forward of the subtank.
you want your primary tank to be the first to contact the hoard.. if you find your sub tank taking damage and not your main, pull them back a little bit it will cause the hoard to focus in on your primary tank which is still standing there... you only need to take a step back..
I hope this helps, if you follow this tips you'll find that medic lockout is not as much of a problem and you'll find it is a very good tactic when dealing with elites.