The bug
Since 1.15-pre4 the debug screen FPS / TPS graphs are not rendered correctly anymore:
Min, max and average values are rendered behind the graph columns now
The "30 FPS", "60 FPS" and "20 TPS" labels are gray(fixed in 1.15-pre5)MC-167105: Graphs behind hotbar, but apparently only when no item is selected in the hotbar
Yeah I also saw this bug some days ago and took some screenshots