Faint red flash on Face ID sensor in Dynamic Island

Higher wavelength IR is invisible. But most manufacturers use what might be best described as near infrared. Serves the same purpose, but under proper conditions, can be seen by human eye.

Specifically it’s in the 800 to 850 nanometer range.

Yep, that is the IR camera. Apple explains that you may see it in dark settings and also when using a camera. The TrueDepth camera is made up of many sensors.

When viewed through certain types of cameras, you might notice light output from the TrueDepth camera. This is expected as some cameras might detect infrared light. Some might also notice a faint light output from the TrueDepth camera when viewed in a very dark room. This is expected in extremely dark settings.

About Face ID advanced technology - Apple Support

There are a couple of “illuminators” in that notch.

Occasionally they can be faintly seen with the naked eye by some individuals in certain ambient lighting conditions.

iPhone cameras will detect them and render them onto the screen display. (They will also detect and render images of IR transmitting devices of many of the multitude of IR-based remote controls)

All in your post sound’s perfectly normal.

The infrared flood illuminator may be visible to the human eye in a darker environment. Nothing unusual.