OK.
I think I found the culprit.
The content pipeline code in the develop branch.
Here’s what happened when I changed the generated XNB files POST compile but PRE load to some really old ones I had lying around in an old project branch (from the time when it still worked).
The thing is that switching the MG releases didn’t help because the CP that gets started when the build process starts is always the same, the one that is installed via the installer, e.g. 3.6.0.1488 in my case.
So the content pipeline seems to get the borders wrong when generating the XNB file.
EDIT: Upgraded to 3.6.0.1551 using the installer. Still no change/same problem. @nkast any ideas when/why that happened?