I looked it up and it seems you're right, so I removed the "adaptation" tag. If you notice errors like this in the future, report the series with "information to correct" and provide some kind of evidence/justification to support your claim. The mods will then fix it for you (may take a day or two depending on how busy they are).
If you run across another tag you're not sure of, there's a working tag definition document here:
https://docs.google.com/document/d/1DzIHLDBQChJK0vRf-1v9PDRi0adPKSZEk4PjPS3wIKg/edit?usp=sharing
Thank you to everyone for your responses.
I am aware of the report function, and have already used it, uh…
quickly checks around a dozen times in the past. But now I can edit things directly¹, so it wasn't an issue of how to change this information but whether it should be changed at all. (Still, might be a useful clarification for any on-lookers.)
My first instinct was to check for a tag wiki or something, but I couldn't find anything; this Google document wasn't anywhere I could find. And it's a good thing you posted it, too!
See, there's this manga (“Kainushi Juujin to Pet Joshikousei”) which abruptly ceased publication 2-3 years ago, without any official announcement. I thought that warranted a “Hiatus” tag, but apparently unless it's been specifically formally announced as such then it's supposed to be marked “cancelled”.
¹ My request for a group take-over has been finally granted! Many thanks to whomever granted that.