Indeed, portamento between "stopped" notes should not possible, the IT compatibility setting "Do not apply any portamento if no previous note is playing" (enabled by default also in MPTM) takes care of that. Previously this compatibility setting did not affect OPL instruments (accidentally). Of course it's a bit unfortunate that this breaks existing OpenMPT 1.28 behaviour so I will see if anything can be done about that.

Oh okay so it isn't supposed to be like that. I tried to extend the envelope duration by increasing the last node by like 128 ticks, however the envelope doesn't reset. I thought that envelope-carry would have to been set for that to happen, or maybe I know less about Gxx than I think?

Envelope Carry is for notes without portamento. If you want envelopes to retrigger with portamento notes, you have to enable "Compatible Gxx" in the Song Properties dialog.

Something I found out right now in asikwp_and_wito_-_midday.mptm at pattern 22 (https://modarchive.org/index.php?request=view_by_moduleid&query=187116), that might be related to this, is that the Qxx seems to be affected too since it wont retrigger any FM samples.

That's normal - ST3 does the same. You probably just didn't realize it while composing. :) Feel free to compare against OpenMPT 1.28, the drums will also not retrigger there.

Fixed in r13339. I also improved retrigger behaviour for MPTM files. This is controlled through a new playback compatibility setting, which is enabled automatically for newly created files.