That auto value is “magic”, which depends on selected dashboard time range. But apparently it doesn’t play very well with Thanos and wider timer ranges. (Maybe you can tweak Thanos to be more friendly to Grafana, dunno, check why Thanos returns 400, so you will know exact root cause).
I guess Thanos has also own, but different “magic”, which calculates resolution.
I would use own time variable (but only in the dashboard, not explore), where you can customize how granular time aggregation should be, e. g. 10,100,1000,… points per graph. Play with that until you reach desired results for any time range.
