If the phones are in use, they probably have a passive connection glitch (first login settings). I explain below by analogy:
the established passive connection between the accessories and the motherboard is established once, during the installation process of the new build of the environment. Apple always uses this model, and that's how it was resolved in the entire family 13, 14 and all earlier. If the cameras were working during the initial launch, were injured (or some connections were interrupted - due to use, drops, liquid spills) the information will be uncovered and displayed only during the next system upgrade or restore. Some of the connections I mention are not required to use the accessory (e.g. wide-angle camera) but only when retrieving the serial number or checking the serial number of the connected sub-device (accessory).
The other cases to mention (and due to Apple's popularity it continues to apply) is intentional substitution. It doesn't always mean attempted theft (although I've had such cases with screens on earlier models before I introduced some security) but just a mess on the table - as with the Home buttons, if you remember similar situations. Technician removes buttons or Face.ID elements from several devices at the same time (to save time), a mistake occurs. Phones that are not inspected return to their owners with buttons and screens that are not their own, depending on what was changed there. It also happened to me many times when worried users came to me that in some repair shop, during the repair, the Home button or the Face.ID function was broken and whether it can be repaired at studionapraw.pl. Upon physical examination, I most often find that the serial numbers do not match. In most cases (it takes about 15 minutes) the owner manages to return to the previous workshop and file a complaint. Many such cases have been resolved positively. Sometimes, however, the previous workshop denies, unaware of the mistake. It happens sometimes.
Due to the fact that, as you write, the phones have been opened somewhere, it is possible to deliberately replace damaged parts from other phones (this can be verified with serial numbers) or - which seems most likely to me: the purchaser found a camera error during the installation of a new user (ios upgrade). Because he has some knowledge on the subject, he tried to solve the problem himself (check if the motherboard has not been smashed - which also happens sometimes). Due to the failure, he returned these things to you with a claim.
Considering what I wrote above, I believe (based on my experience with similar cases) that these phones have a fault that has not been discovered before. The primary fault must be found and removed, and the operation of the cameras, sensors, microphones installed in the phone will return to normal (if it has not been demolished before by homegrown specialists of the YouTube era. Be sure to check it out.
More similar cases can be found on my blog (check my description), where I help affected users as a non-profit through mostly dishonest repair shops (sometimes even authorized) from around the world.
- regards and fingers crossed