Single replica cannot flush data when transientStorePoolEnable is true in 5.1.0

[RongtongJin]

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5. Work out a solution to rectify the behavior and make the newly created test cases pass;
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As a result, it would be very helpful yet challenging if you could provide an isolated project reproducing your reported issue. Anyway, please ensure your issue report is informative enough for the community to pick up. At a minimum, include the following hints:

BUG REPORT

1. Please describe the issue you observed:

• What did you do (The steps to reproduce)?

Single replica cannot flush data when transientStorePoolEnable is true
This bug import when fix #5722

• What is expected to see?

• What did you see instead?

2. Please tell us about your environment:

3. Other information (e.g. detailed explanation, logs, related issues, suggestions on how to fix, etc):