Bug when pulling subtree changes

Hello,

currently I suffer of a strange bug with subtree.

Long ago, I did the following:
   git subtree add --prefix=stdlib https://github.com/puppetlabs/puppetlabs-stdlib 4.12.0
   git subtree pull --squash --prefix=stdlib https://github.com/puppetlabs/puppetlabs-stdlib 4.19.0
   git subtree pull --squash --prefix=stdlib https://github.com/puppetlabs/puppetlabs-stdlib 4.20.0

Today I wanted to do the following:
   git subtree pull --squash --prefix=stdlib https://github.com/puppetlabs/puppetlabs-stdlib v6.3.0

But got the following error:
   From https://github.com/puppetlabs/puppetlabs-stdlib
    * tag                   v6.3.0     -> FETCH_HEAD
   fatal: ambiguous argument '3a4414eb087da0dafa147df047f6bf8f18a6a621^0': unknown revision or path not in the working tree.
   Use '--' to separate paths from revisions, like this:
   'git <command> [<revision>...] -- [<file>...]'
   could not rev-parse split hash 3a4414eb087da0dafa147df047f6bf8f18a6a621 from commit c1118a27ef27c1fd4df7877f0b0fb606ea30f0fd
   Can't squash-merge: 'stdlib' was never added.

The revisions are fully valid:
- c1118a27ef27c1fd4df7877f0b0fb606ea30f0fd: The last squash revision in
  the private repository
- 3a4414eb087da0dafa147df047f6bf8f18a6a621: Tag 4.20.0 in
  puppetlabs-stdlib repository
- Tag v6.3.0 pointing to 990cd99 in puppetlabs-stdlib repository

The current git version is 2.27.0. I have no record about the revision used for the pulls in past.

Anyone an idea why this doesn't work anymore?

Please keep me in Cc as I am not subscribed to the mailing list.

Regards
   Klaus