Time/date handling is all very relative. Depending on where you live, the first day of week is sunday or monday and midnight maybe noon. Roon has a somewhat long history of features missing time conversion algorithms. As this is the first implementation of this feature, I wouldn’t wonder if it just uses the raw UTC timestamps to fill the graph, missing the needed transformation to local time (to get it right for the installations TZ) before doing so.
Nice catch if there is indeed an error (I live to close to GMT to notice anything in the regards I described above).
Update: Just noticed that you’re talking about the pop-up tool-tip only - so my concerns here are maybe just paranoia caused by past experiences and possibly unrelated.