Is that normal and expected mobile functionality? I'm not aware of anything similar that provides tooltips when holding buttons, could you give examples of apps that do?
If such tooltips are added, please enable them on hover too. This would work with stylus (and mouse), of course, but also e.g. Samsung phones have a built-in option to enable hover functionality with fingers.
I've seen it in a calendar app long time ago (to display event details in a month view), but it's probably not very common.
Fair enough, don't think I've ever even tried to press and hold in that context before, to me it has always been a metaphor for a context menu à la right click (the F-Droid one seems particularly useless seeing as it already has the labels?).
I tried it on a few more Google apps, Gmail, Drive etc. and is part of Material Design so I have no issues for the Android implementation of it personally. - Material Design