Since you’re below 61 this does sound like a bug.
I’m having similar problem in WotLK Chromi time but at level 65. My list of dungeons shrunk to 2: Nexus and Utgarde Keep; which is likely a bug as there shouldn’t be any anymore. We can still play content just not dungeons – except through DUNGEON TW per Blizz calendar.
Luckily I already bagged the WotLK Dungeon Master achievement on this toon, but I run into this problem with every toon in Chromie time; where I’m still enjoying the experience and have lots more to do but reach level 65 way too soon.
It’s a disappointment because it ruins the continuity of the experience and robs me of the timely gear. And I forget there’s this dungeon wall, where you cannot even walk in on foot.
For example: at the entrance to Naxxramas I receive a chat "You cannot enter this instance while participating in a Timewalking Campaign.
Now that’s a kick in the teeth; especially since I always forget this and struggle with bug reports and forum searches each time it happens.