tidb_service_scope won't take effect after 90s

[ywqzzy]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Deploy tidb with default config.

set global tidb_service_scope=background;
select * from mysql.dist_framework_meta;
# after 2minutes
select * from mysql.dist_framework_meta;

2. What did you expect to see? (Required)

tidb with background scope after 2 min.

3. What did you see instead (Required)

tidb with scope "" after 2 min.

4. What is your TiDB version? (Required)

v7.5