The front camera (selfie) can be affected by an improperly fitted replacement screen or the camera bracket It can also be caused by smudges or tape or anything else on the backside of the digitizer that wasn't properly cleaned or removed prior to final assembly.
Did you notice the problem immediately after the repair or did it work for a while? Normally, a repair shop should test all functionality before opening the device and once it is repaired. Sometimes, there are other failures besides the cracked screen that the customer hasn't noticed or mentioned. If the technician makes a mistake or forgets something, then a final checkup before sealing the device can catch it.
In this case, it sounds like one of those two scenarios occurred. If the shop truly made a mistake, they should cover it under warranty. If you didn't tell them (you say you did) or didn't give them the passcode so they could test all functionality, them some of the fault lies with you.
Only you know what was said and done. Talk to the tech in person and figure out the best solution.