After deleting pool members, traffic was still forwarded on them.

Can you provide an output of the show sys connection ?

Also you see from the GUI web interface that the pool members are removed ? If so check the CLI "tmsh show running-config" and see if the pool members are under the VIP, you may also check the config file "/config/bigip.conf" (if you have partitions there will be such file for each partition). If you the pool members to be removed from the GUI and bigip.conf but not the running config, do load sys config verify to see for errors. If there are no errors you may try mcpd reload for issues with a different config between the data plane and control plane:

https://support.f5.com/csp/article/K13030

If you still have issues after everything check the bug tracker, for example and maybe open a TAC case if you don't find anything in the bug tracker

https://support.f5.com/csp/bug-tracker?sf189923893=1

For first time I heard for such issue with the F5 device, so if you are right and not missing domethink obvious in the config (like checking the wrong VIP in another partition that has the deleted members, etc. ) as it is really strange.