That is, until you exit the cell after picking up the companion. At this point, the companion goes into combat mode against enemies that aren't there, and cannot be fixed. The best you can do is use the companion's summoning ring to bring them back to you and very quickly open an interaction with them (within about 1.5 seconds), and you can "talk" to them (use their menu) again, but as soon as you exit the interaction, they return to paranoid freak-out mode. This is best tested with Beryl or Hurd, since the Wolf just runs around; Beryl and Hurd issue combat dialog and attack the air.
This may relate to problems people have had with other companion mods by Emma (best known as author of Children of Morrowind or CoM), such as Laura Craft Romance and Adventure, and The Witchgirl Adventure, mentioned on the Mod status#List of Catalogued Mods wiki table. Not much detail provided, and both listed as blue icon (in need of testing since last attempts to resolve issues with them); I don't find much about them in the forums. The mods I'm testing are not listed on the Mod Status page.
I don't know anything about the "Julan, Ashlander Companion" mod, which is said to be working now with a bunch of work-arounds, so I have no idea if the stuff that's making it mostly work would also work with Grumpy/Emma companions. Given how many companion mods are based on Grumpy's code, that's possible.
- I'm using English-language OpenMW 0.41.something, the OpenMW-controller-fix-9e8e9aef8.dmg version of 28-Jan-2017. I'm not sure exactly what version it is, since when you do "Get info" in the Finder, the Version field is just "--", something that should be fixed; so should the fact that when OpenMW is running, it's main system menu, "OpenMW" has no "About OpenMW" option, another expected way to get version info. The bottom of the GUI says "OpenMW development (9e8e9aef8).
- System is macOS Sierra 10.13.3, on a Mac Pro Mid-2010, 12-core, with 48 GB RAM and enough disk space to store all of Texas.
- This bug triggers, 100% of the time, with whatever character I test it with; with no other mods loaded but the stock Bethesda ones from the GoY edition (Tribunal.esm, AreaEffectArrows.esm, etc.), all vanilla except the one companion being tested; with clean savegames; and a clean Morrowind directory. Tests I did with UCS and RPP were in a different copy.
- The mods are being added the old-fashioned way by manually moving the files into the Morrowind/Data directory and subdirectories thereof.
PPS: I know jack about game development and modding; to the extent I'm a coder at all it's bash, Perl, PHP, JS, and a bit of Python, Ruby, and Lua. I haven't even used OpenMW-CS yet, and as a player not a dev I haven't had much interest in delving into it, though I'm of course willing to do stuff with it to get stuff to work, if necessary.
PPPS: I have not tested a "pure" Grumpy companion, but I don't think Emma did much to his code after his death, just built new companions on his widely available template (Companion Project 3.1). That barebones companion codebase may be useful for debugging this issue.