Did you try to load some chunks of it in a 1.8 version and then reopen it in 1.9?
It might've messed up the world a bit.
The comments on MC-74103 are addressing that yes, it can cause lag and that is normal, but they have not addressed that there is no way to undo this action as it freezes all mobs - I believe it can still be called a bug because after the mob is spawned the world will be permanently altered.
I believe this is intentional, it's supposed to allow an infinite effect to be set with a high count.
I doubt this would matter but it's always safest to use a parameter for seconds in addition to another one for the effect level
You could also do:
/entitydata @e[type=Sheep] {Glowing:1} for a permanent glow
Sorry, I thought marking as 1.9 encompassed snapshots, I'll fix that now
The bug has been known for a while - since 2012, actually, according to that ticket - yet it's still in the game. I was hoping that by making a new ticket I would actually get the bug fixed.
I understand that it's challenging to fix these things, but it has been around for three years.
I really think this doesn't work as intended, I'm having similar issues one year later. Using /entitydata to update the tag also does not change the dimension, this seems to be an actual bug.