Apple iPhone 7 plus stuck in bootloop after screen replacement help.

First of all, never assume a screen is good just based on the suppliers reputation. Even the best suppliers ship out duds occasionally. The best suppliers will keep this to the low single-digit percentage while bad suppliers may ship out 10-20% (or more) junk. When technical folks here talk about “known-good” screens, they mean screens you KNOW work properly, not think/hope/assume they do :>).

Now the next thing is the condition of the connectors. They are easily damaged and depending on the type of damage, it may be sufficient to cause failures. You need to look at them under magnification, something like a jeweler’s loupe is good and cheap. If there is too much damage, it will have to be replaced either via micro-soldering or the whole flex.

Finally, did the replacement screen come with the Front Camera Flex pre-installed or did you transfer it over from your original screen. It’s not uncommon for pre-installed FCAM flexes to be poor quality. It’s always better to transfer the original and maintain as much “original” components as possible.