It's deterministic, you've just changed the inputs. The "committer date" is usually excluded from logs so you don't see it, but it's the only missing input. Here is a demo:

    $ git rev-parse HEAD 
    78d66a4f169fec9cb9b3252f7a22bb020e967cd2

    $ git log HEAD      
    commit 78d66a4f169fec9cb9b3252f7a22bb020e967cd2 (HEAD -> master)
    Author: XXXX
    Date:   Mon Nov 2 20:44:04 2020 -0500

        temp2

    $ git reset --soft HEAD\^

    $ GIT_COMMITTER_DATE='Mon Nov 2 20:44:04 2020 -0500' GIT_AUTHOR_DATE='Mon Nov 2 20:44:04 2020 -0500' git commit -m temp2
    [master 78d66a4] temp2
     1 file changed, 0 insertions(+), 0 deletions(-)
     create mode 100644 b

    $ git rev-parse HEAD
    78d66a4f169fec9cb9b3252f7a22bb020e967cd2