[TYPO3] Incomplete MP Variable after upgrade

[Frank]

Hello List,

We've recently upgraded a typo 3.8 setup with 50+ websites running from it
to 4.2.1

This worked better than I hoped for the most part, and here is a last bit
which i can seem to put my finger on:

We have a few scenarios where a subtree from one site is mounted via the
Mount-point page-type into the tree of the other side. In the source-side
(of course) everything works, but the navigation points in the second site
have an incomplete MP variable:

/index.php?id=247&MP=232-

which will lead to an error:

The requested page didn't have a proper connection to the tree-root!

If I manually extend the link with the proper id (the page id of the
'mount-page') then it works as expected:

/index.php?id=247&MP=232-5534

the page is now shown.

An interesting part here is if i manage to get into a sub-menu via the
resolved real-url path, then it works without a problem.

any hints or pointers what here is amiss?

thanks in advance
Frank