Gradle 7.6 upgrade doesn't upgrade gradle-wrapper.jar

Followed the upgrade instruction per the release note: Gradle 7.6 Release Notes

./gradlew wrapper --gradle-version=7.6

After checking gradle-wrapper.jar 's SHA256, it still shows 91a239400bb638f36a1795d8fdf7939d532cdc7d794d1119b7261aac158b1e60 which is the checksum of 7.5.1 or 7.5

Is this a bug from 7.6?

No, that’s expected, documented at The Gradle Wrapper, and the same since ever:

Note that running the wrapper task once will update only, but leave the wrapper itself in gradle-wrapper.jar untouched. This is usually fine as new versions of Gradle can be run even with ancient wrapper files. If you nevertheless want all the wrapper files to be completely up-to-date, you’ll need to run the wrapper task a second time.

The wrapper jar and the start scripts are always taken from the version that currently runs.
So if you run the given command, the wrapper properties file is written to use 7.6, but the other three files stay untouched unless you run it a second time.