Some phones will turn their screen off when the top infrared proximity sensor is triggered when on a call, since people usually hold press their phones to their ears which will interact with the screen and interfere. Have you placed some tape over the front camera area for privacy? Is the top area clean? Try seeing if there is anything that could be obstructing the top area and causing the proximity sensor to misfire.

The problem will likely persist even after a long while of drying due to the fact that the Galaxy S21+ should have a water resistance rating of IP68. While the IP68 seal may have been compromised at a small point (otherwise there would not be any water ingress), it will take a very long time for all of the moisture to evacuate your device naturally from that tiny, localized break in the seal. I would recommend following this procedure to take the camera out. You can then let it dry for a couple of hours or very carefully use a piece of paper to soak up the droplet, trying to limit the number of fingerprints and amount of dust that may fall onto the surface of the components.

We are unable to see what you mean with the screenshot since it appears to be perfectly normal. This proves that this is a hardware-side issue since the software is not purposefully causing the flashing and thus cannot capture it in the screenshot. I would advise a screen replacement, which can be done with the following procedure. Without more information or a look on the inside of your device, we will not be able to suggest many other options.

I would advise 60-110 degrees Celsius to soften adhesives. Be careful to not overheat components when operating at the upper quartile of the mentioned range.

My best guess would be that the pins of the ribbon cable could've been damaged, or that power was sent down the incorrect channels when it was flipped, damaging some components.

Odd. Have you been using the same supplier for the XR replacement screens? I would usually say it might be a loose digitizer ribbon cable that hasn't been seated correctly, but since it has happened twice and it worked perfectly for a while, I would point a finger at the quality of the screens.

The WiFi card seems to be integrated into the motherboard as suggested by Step 9 in this guide where the antennae coaxials terminate at the motherboard.

I wouldn't advise doing so since moisture is quite stubborn and will often just rather condense inside the case. Ideally I would use isopropanol in the ultrasonic bath, but that is a significant fire risk that isn't worth taking.

You will likely need to buy a replacement screen and follow this procedure. The screen should come with the touch digitizer which seems to be the problematic component in your device.

The repair will not be easy. By the looks of it, you will need to tear it down completely (this is for Gen1, but Gen2 will likely be similar) using complex tools since the Apple Pencils are sealed shut permanently. I am currently on mobile data so I can't watch it, but I have found this video that seems to go into the procedure. You may need to buy a new plastic body since you might have to cut into it to be able to repair it.