You all might recognize me from the Skyrim bug reports about the plant and enemy respawns.

I will confirm that the plant life in Solstheim will also not grow back after thirty days even if you leave the island. I have left and waited in a building on mainland Skyrim for three in game months to test this .

Also the enemies that respawn in locations on Solstheims wilderness do not respawn. I also waited three in game months to test this bug on mainland Skyrim in a building.

This is with a forty hour character I made to test 1.8 and Dragonborn. It seems the 1.3 respawn bug has effected the expansion as well.

Thanks for bringing this up. I'm on the PC, but this something to check out to see if it affects all versions. The bug was a silent killer.