I just did the replacement screen (LCD only) on a 2018 model a1989. Everything works perfectly and the reason for that is probably because I kept everything original and replaced just the LCD. I’ve heard rumors about the T2 chip causing issues for repair and most likely that is your problem. The only way to keep True Tone would be to keep the front camera flex (most likely this is where it’s paired). One way to test this and confirm would be if you still had your original LCD assembly and you can just plug in the front camera flex which is the connector closest to the LCD connector on the LCD board. There should be a somewhere that says SV_CAM (on mine it also says J0802) Plug it in, boot up and see if everything is working perfectly afterwards. Good luck
I have personally never seen the issue (what caused it) but I did have a look at another one from a shop that brings me stuff they can't fix in hopes that I can fix it. Lol. This one had some very slight water damage probably like 1 drop next to the charge port connector. Anyways I took a quick look in a 2 day period I could not find the cause. Everything checked out. I changed out the sketchy looking capacitors. I even changed the tristar and still same issue. Would not charge. Good luck maybe if I have more time with one in the future I may be able to figure it out.
Yep it's very likely a faulty (shorted) power button. Don't let this go on for too long as constant reboots can and will damage the storage portion of your phone. If it goes on for too long the software will corrupt and the storage won't mount and you'll have a bricked phone.
This would be pretty straight forward fix for any microsolderer. If you bring me just the board I'd do the work for no more than $100. Not promoting any services. Just saying. I'm currently not in front of schematics but it looks like this could be the culprit of your issues.