It's there for apps that don't play nicely with GrapheneOS. The app is still sandboxed, it just doesn't benefit from extra exploit protections native to GrapheneOS.
Discussion
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
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.