As the title told.
This seems to be broken after an upgrade of the homeassistant core.
That don’t happen on the corresponding temperature card.
Data reading is correct.
It’s not flat.
Likely, the scale is 0-100% so the variations are less noticeable that in the graph below.
Yes, It is definitely not completely flat.
But the wrong scaling still cause bad visablity and should be fixed.
I know it’s probably not what you’re after, but you could use this custom card with way more options: GitHub - kalkih/mini-graph-card: Minimalistic graph card for Home Assistant Lovelace UI
I’d argue the scaling is not wrong, it’s just not labelled; and with a data point every 10 minutes (detail: 2) the “noise” of the “real” graph is not apparent.