If you look at YouTube V3 Docs it will say
YouTube uses playlists to identify special collections of videos for a channel, such as: (...) watch history (...).
Now, if you go to Channels List API and make a call for part=contentDetails&mine=true it will list your information but watchHistory will appear as HL and watchLater as WL.
It makes me wonder, and so far I couldn't find any explicit mention, from Google/YouTube, were those playlists removed from API? IS there any way to actually get that information from an authenticated user?
For quite some time I observed the same values you mentioned on various channels. According to the docs, watch history and watch later playlists are deprecated.