env.GIT_COMMIT is overwritten when pull lib （lib and code in the same repo）

SharedLibrary put in the same code repo, different branch named xxx_lib, and after Declarative: Checkout SCM, can not get the right GIT_COMMIT, always be the xxx_lib's last commit, not the code's commit

I create example repo to this issue: https://github.com/bonfy/JENKINS-61317

Jenkinsfile in master branch Last commit: 39f25e1a
library in jenkins-lib branch Last commit: a8c235b1

and when echo GIT_COMMIT, it is overwitten by lib's commit

16:55:47 stage 1
[Pipeline] echo
16:55:47 GIT COMMIT : a8c235b10c55e2f065293b1c1ad9b3bffd45d9d9
[Pipeline] echo
16:55:47 GIT BRANCH : jenkins-lib
[Pipeline] echo
16:55:48 Hello world