THX a lotHi-
I had the exact same problem just start yesterday.
So here's what I did:
1. download the Magisk 1.8 found here in this forum: title is "Magisk 18.1 for Mi Mix 3 available"
2. Follow these instructions on how to install it (google "how to install magisk XDA")
(essentially install the zip through TWRP and then the Magisk Manager)
3. Then after everything is installed, go into the manager and under the menu on left choose Magisk Hide and choose Google Pay
4. Then you have to go to your Apps manager, clear data and cache of Google Pay and Google Play Services
5. I rebooted just for good measure
6. then I had to re-add all my cards and this time it let me (before it was saying I can't re-add my card because my phone was rooted, etc)
So is this a permanent fix or is it only temporally?This works perfectly for me.
1) Boot into TWRP, delete "/data/data/com.google.android.gms/"
2) Boot to System
3) Delete GPay App Data (Cache & Data)
4) Start Gpay Once an close it right now, without adding or changing something
5) Boot into TWRP
6) Note the name of Folder inside "/data/data/com.google.android.gms/app_dg_cache", only one should be here
7) Boot to System
8) Run SafetyNet App, should be green and no errors
9) Boot into TWRP, now there are 2 Folders in "/data/data/com.google.android.gms"
10) delete "/data/data/com.google.android.gms/<FOLDER FROM POINT 6>", the other Folder should stay untouched
11) CHMOD 0551 to folder "/data/data/com.google.android.gms/app_dg_cache"
12) Reboot to System and set up GPay with my Paypal Account