I seam to have a bug with the steed mundus stone effect is randomly removed after completing random dungeons via the group finder, I have had to reapply it multiple times, as it becomes absent after one to five dungeon runs.
I have tried other mundus stone and the don't seam to have this problem, and have now switched builds to avoid this problem, but I thought I would mention it here.
The character was originally a DC Breton templar created some 4 or 5 years ago, that was then changed to a DC Nord templar about 2 years ago on PC.