As the Github pull bot doesn’t support syncing the tags form the upstream repository the snap will always stick on the last release when the pull bot is incorporated. This should be fixed.
$ snap info youtube-dl
...stripped...
channels:
stable: 2018.12.09+git347.495bf63 2019-02-25 (711) 88MB -
candidate: ↑
beta: ↑
edge: 2018.12.09+git403.6e93ef5 2019-03-10 (811) 88MB -
The official release version string can be acquired from youtube_dl/version.py, the generation of the proper commit depth since release is rather complicated, though.