I’ve successfully charged my FW16 off the two official chargers, an old 100W Hyper Juice charger, a less old 100W Hyphen-X charger, an 87W MBP charger, and a 100W battery pack.
It won’t charge from a Steam Deck charger, that I’ve seen. Guess 45W doesn’t cut it.
I’d thought that feature was an Apple original – to placate users, the hardware does what it wants and rounds up a few percent on the charge level. I’ve got a MacBook with two percentages in the tray, the actual percentage from a third-party app, and the official, higher, percentage from Apple’s own battery widget. I’ve never liked that kind of hidden-complexity / dishonest UI myself. iPhones do it too.
… poking around the forum a bit, it looks like there’s a fair number of known issues with various chargers, especially from these two threads:
this post from that thread sounds a lot like what you’re describing, has to do with USB cable interactions. What a mess.