Turning on this mode did not fix my issue... Always gives an error that "they could not confirm this account belongs to you". This seems to be a known issue on the GrapheneOS forum as well
Discussion
I don't have to enable it on my Pixel 6 Pro, but if I recall correctly, I was able to help someone before with this advice. Sorry it doesn't help with your device/situation.
I have used CashApp on GOS for years without issue on my Pixel 6 Pro.
I just uninstalled and reinstalled to verify, and all is well. It prompted me to installed a library with the app. Maybe try reinstalling and see if you are prompted to install the library as well. Beyond this, I am afraid I cannot help.