Nintendo has addressed a glitch on the Switch where the battery percentage displays incorrectly or not at all. The new update should fix this, though. This is good news.
What may not be good news, from what some sources suggest; to let the Switch run down to 0% several times in a row and then let it sit on the UI for several hours. Hopefully, this part isn’t the case for everyone. It does, however, seem to fix the issue.