The acceptable (by standards) range for the main voltages of a PSU is +\-5%. So for the 12V is 11.4-12.6V. The ideal is to stay as close as possible to the rated voltage. 11.9V is 1-1.5% off and it’s not all that bad.
If your sensors window has other 12V readings check them too. There is a possibility that those 11.9V readings to be just fault readings by GPU sensors.
Or you PSU might be multi 12V railed and the GPU loads heavy the one of them and you see that voltage drop.
It’s nothing really to worry about, just keep it under track.
Whats the PSU brand/model and what GPU you have?
I appreciate the in depth reply! And, sorry for my late response! I did know that they were in range, but seeing them frequently change scared me. There's also a 12V reading under the motherboard section, and it seems to stay a bit more constant than the 8-pin ones.
I have a 3080 Aorus Xtreme, and an 850w platinum be quiet! straight power 11 (what a mouthfull, haha), which is multi-rail. I've actually been stressing if I have it hooked up in the most efficient way, and am wondering if that could cause issues, too.