log backup lag reached 15 when random tikv shut down

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1. 10tidb(16C32G)-3pd(4C8G, 20G per)-18tikv(16C64G, 2000 per), 10T data
2. enable log backup
3. run workload, and make one of tikv failure 10 minutes

### 2. What did you expect to see? (Required)
log backup log within 10 minutes

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)

Release Version: v8.5.3
Edition: Enterprise
Git Commit Hash: dcbfb10cd28823999e66aae541cdc5d904d231a6
Git Branch: HEAD
UTC Build Time: 2025-07-21 17:07:22
GoVersion: go1.23.8
Race Enabled: false
Check Table Before Drop: false
Store: tikv
Enterprise Extension Commit Hash: 7d43ff65ebc145bd63fa84cb368f8775be906998