Magnetic Case Affecting iPhone 17 Pro Max Compass?

It potentially can.

I use MagSafe cases myself on my iPhone - currently I use one from the Caudabe brand. I have also used Spigen cases before, as well as cases from Nomad. I have also used Apple's original MagSafe cases, and I have never experienced problems with the compass with any of the above brands.

If you want to be on the safe side, you should choose an original Apple MagSafe case, as these are 100% adapted to not interfere with the compass.

AI says:

Yes, a protective case with a magnet can affect the iPhone's internal compass, particularly if the magnets are strong or poorly designed. While MagSafe accessories are designed to minimize interference, non-MagSafe magnetic cases can cause inaccurate compass readings or other sensor issues.

I’ve always used Apple’s MagSafe cases, but one day I came across this site called CForCover.com and found such a premium case that I thought, let’s go for style this time instead of just protection. Anyway, I haven’t faced any issues with this case so far.