Wandering NPCs falling through cantons

Morrowind GOG edition
OpenMW version 8779d261-win64 2020-03-25 (latest nightly)
Windows 10
I will give full mod details at the end

I encountered a bug which I think is related to #3130 (closed) and #1867 (closed) (at least in behaviour, obviously I can't tell about the cause).

New character, started in Ebonheart (no issues there), got to Vivec, changed by Redesigned Vivec.
Started the slander quest in the Foreign Quarter: got the leaflet from the wandering redguard, Domalen, in the plaza, talked with the alchemist, went back to talk to the redguard but I was unable to find him.
Via ra I had him get back to his position, but as he was in swimming stance when he reappeared, I started investigating.

Whenever I change cell via a door near Domalen or whenever I (quick)load a save he falls through the meshes and into the water (underwater). He doesn't fall if I use a door "far enough" from him or if I load a save "far enough" from him.
I hoped that this behaviour was limited to a few NPCs in the area, but further testing made me reconsider.
Testing a few other doors around Vivec, I found that the same happened with the Ordinators that should be stood near the (northern) doors of the Hall of Wisdom and Hall of Justice. More worringly, a few NPCs who I thought were unaffected, fell when they got near one of the doors I used for testing and I used it.

tcl and tai have no effect (to be precise, toggling AI off causes the NPCs to drown).
I didn't notice occurrences of the bug in Ebonheart (which is why I mentioned it above) despite me staying there a while.

I tested this with my full mod list, with Morrowind, expansions and Redesigned Vivec and with just Morrowind and Redesigned Vivec.
On Morrowind GOG edition, with expansions and Redesigned Vivec only the behaviour does not occur.
The mod has been cleaned with tes3cmd but the bug occurs with the "dirty" version too.
Without the mod I haven't been able to replicate the behaviour.

In my humble opinion, I don't think that Redesigned Vivec is the culprit here. I suspect that it simply makes it easier for the bug to happen as it places many NPCs on those big canton models (more than Molag Mar and other vanilla locations).
If this were the case, the same could happen with Almas Thirr in Tamriel Rebuilt, but I haven't checked.

Some screenshots

CS placement of the NPCs

Domalen starting position in-game

The doors marked with the red dot are the ones I used to have NPCs fall. The bigger building in the upper area is the one I used to have more NPCs fall when they got near. A cursory check on TES-CS shows no issues.

Guilty doors

Back from a swim

NPCs get completely underwater after a fall. I didn't check thoroughly but it seems to me that they go down until they reach the land underneath ("terrain level" may be the term?).

Underwater

Pool party