Topic subscription backlog can't be cleared

**Describe the bug**
Pulsar v2.6.2
Use clear-backlog command:
bin/pulsar-admin topics clear-backlog ***
Topic subscription backlog can't be cleared.

Use unload command:
bin/pulsar-admin topics unload ***
Topic subscription backlog can be cleared.

But I don't know if any messages are lost, and why the clear backlog can't be cleared. Is it a counter error?


**To Reproduce**
Steps to reproduce the behavior:
1.start consumer, receive msg
2.reconsumerLater msg

Start by reproducing the issue with the `pulsar-admin topics clear-backlog` and `unload` commands, following the consumer and `reconsumerLater` steps in the report. Trace the topic subscription backlog handling for both commands and verify whether clearing the backlog removes or loses messages. Done means the behavior is explained and the clear-backlog path works as intended with regression coverage.

Written by the indexing model from the issue text.