Very interesting! I hope you find the cause…
I did find these settings of how to supposedly disable IPv6 on Android devices, but all Android devices are different.
Go to your Android device System Settings and tap on “Network & Internet” (1).
Tap on “Mobile network” (2).
Tap on “Advanced” (3).
Tap on “Access Point Names” (4).
Tap on the APN you are currently using (5).
Tap on “APN Protocol” (6).
Tap on “IPv4” (7).
Save the changes (8).
On my own phone when I got to step 6 the option to change it was greyed out, but perhaps your phone will not have this issue.