git rebase --continue after solving conflicts doesn't work anymore

I've always used "git rebase --continue" as the help shows:

    Resolve all conflicts manually, mark them as resolved with
    "git add/rm <conflicted_files>", then run "git rebase --continue".

and Git would apply the conflicted commit without issues and the rebase would continue.

But since Git 2.20.x it doesn't work anymore. Now after solving the conflicts and running "git rebase --continue" I get this error most of the time:

    error: unable to dequote value of 'GIT_AUTHOR_DATE'
    error: you have staged changes in your working tree
    If these changes are meant to be squashed into the previous commit, run:

      git commit --amend

    If they are meant to go into a new commit, run:

      git commit

    In both cases, once you're done, continue with:

      git rebase --continue

    error: could not commit staged changes.

Now I have to run "git commit" first to commit manually and then do the "git rebase --continue".

I've compiled 2.18.2 and 2.19.2 to check and the error doesn't happen.

It is present in 2.20.x and current master v2.21.0-rc1-9-gca1b411648.

Is it a regression because of the new rebase written in C?