A reminder of Valve’s FAQ, given recent article on Websites trying to contradict (e.g. https://www.slashgear.com/1314340/steam-deck-plugged-in-overnight/):
If your device has been left on the charger for long periods of time, it may report less than 100% charge. This is normal. We allow the battery to slowly discharge after long periods of time under charge to optimize long-term battery health.
Figured so, I keep it in a dock as a spare office PC so I would think they thought ahead with stuff like that.