4G router data deactivated, but it keep backing to 4G (don't stay in 3G)

Hello Experts.

I saw strange behavior while doing some special testing.

When I deactivate the data in 4G router, it goes to 3G for some milliseconds and then back to 4G.

The priorities in last RRC release message didn’t take effect after resuming the 4G connection.

Any suggestions?