Had problems yesterday with getting dismounted on my DK, tought first it was rapid manouver that was bugging, so I tested on another character and had no issues.
Logged back on today to find the bug remained on the DK, so did further testing on both the DK and other chars.
Same thing happens on multiple tested characters, it seems to only happen when changing from the weapon with the poison equipped.
It occours on swap number 1 or as high as 24. Some times there is a dismount sound from the mount but I do not physically dismount.
Tested on dual wield, 1h+shield, 2hand and some random combinations, also on unenchanted weapons. It seems pretty easily reproducable.