When switching items in the hotbar, hearts and armor points are not showing. You have to toggle the GUI with G in order for hearts and armor points to show. Running 1.21.4

This is intended default behaviour. If you want everything to appear together, set "Reveal Type" to "Combined" (everything together) or "Grouped" (the main HUD at the bottom together, but scoreboard/status effects/... on their own).

If you are on one of those settings already, then this is actually a bug. Could you confirm whether this is the case?