iPhone 13 pro max camera glitsching

As nearly as I can tell, there were some early issues with the rear cameras and compatibility with third party apps that use the rear cameras, but Apple apparently was able to address those problems with a software update, although if the issue was allowed to go on for too long, it appears that damage could occur to the camera necessitating replacement.

However, assuming you've done a software update at some point since the phone was released, you should have the fix already installed so chances are that's not the issue - not to mention the fact that it just started, of course.

While it would be worth your time to unplug the cameras and check the FPC connectors on both the motherboard and the flex cable, there isn't a high likelihood that's the problem. With the brackets used to secure all of the plug connectors on an iPhone, we almost never see issues where a plug has come loose. Nevertheless, I would open it up and use a magnifying glass to examine the connectors, then some 90% or higher concentration isopropyl (rubbing) alcohol to clean both sides. Reassemble and retest.

If you're still having problems, it may well be time to replace the camera. Note that all three rear cameras come as a unit, so you'll be replacing all of them at once. Here's the iFixit guide to that repair, although I'm sure you already know all about it if you've done a housing swap and gutted the entire phone.

iPhone 13 Pro Max Rear-Facing Camera Replacement - iFixit Repair Guide

It looks to me like you will most likely get a non-genuine parts warning when you do the replacement, but functionality should be unaffected. If it bothers you and you have the JC device programmer, it seems there's data that can be copied from the original camera to the new one to get rid of that message, much the same way screen data can be copied to restore True Tone function. Here's the article I ran across describing it.

Fixed "Unknown Part" on iPhone 14 Pro Max Rear Camera-No Soldering

That's all I've got for you; hopefully that's enough to get you going, but if not come on back and check in again; there are an awful lot of very smart people on here and one of them will probably know the answer.