Good question. I have the same problem with flatpak (I have both 525 and 515 version installed at the moment) but I use the deb version of steam, not the flatpak one. I should try to remove them.
BUT, if I recall correctly, I still have something relayed to Nvidia 515 installed via apt (for example, when the kernel updates, I see that apt compiles the modules related to 515 together with the ones for 525).