If you go outside the room, use the teleporter pyramids to return to the wastelands (assuming you left one in town) and teleport back, does that get the cutscene to trigger when you enter again?
The only other issue like this that I recall was due to not defeating all 5 of the Black Ring bosses, and using the teleporter pyramids to bypass the spikes that block off the path leading north to the Demon of lies area.
In that case, it should help to return and finish off the remaining Black Ring boss(es).
If not, the easiest fix may be to use
DAD's item editor (broken link) to give a weapon some large damage and stat boosts so you can quickly get through the required fights, run past most of the opponents in the wastelands / dungeon (a frog statuette or ghost form would help), and then switch back to a normal weapon when you get back to the area before the final room.
Backup your existing quick saves, and maybe make a copy of your last named save before editing. Also, the item editor may not work with saves in the Steam folder, but should if you move/copy it to another location first.
If you zip your latest save and email it to me (raze@larian.com), I can take a look at it. Since it is the very end of the game, there is quite a bit of freedom with what can be swapped or edited to try to get it working.
Here is a mirror link for version 1.4 of
DAD's Item Editor.
If you do not already have the Visual Basic 6 redistributable installed, and get missing file/component errors, use the
full installer for v1.1, then replace the executable in the install folder with the latest version.