I didn't login for 3 days now (due to work), and I noticed a huge bump on the graph (the rank still didn't update though, as I didn't login and play). My profile says rank #651, graph says rank #828 and global ranking says #648.
Probably just a recalculation again, but shouldn't that be announced at least? (this graph is for taiko btw)
P.S. checking some random players, they appear to have similar spikes too, however they have the graph going up instead (gaining ranks).
EDIT:
Further observation is making this look like the graph is trying to restart it's initial value @90 days ago, so it could just be a visual error and possibly a separate issue.