I have the no dragon respawn issue. My world was created in 1.8 and I tried to respawn it for the first time in 1.11.2. No luck.
I used an MBT editor as suggested in the workaround comment linked above. I have situation 3 (The world was affected, and currently has <DragonFight> but not <ExitPortalLocation>) but the comment claims that this is now fixed, but I still can't spawn the dragon.
I attached my level.dat and a screenshot of the portal.
Perfect. The workaround suggested worked. Found a way to break the portal in survival and respawned it. Now dragon summoning works fine.
Much appreciated. Would like the bug to be fixed for others though as its taken a good few hours of searching and trying different things for me to get this sorted.