The left-hand sensor isn’t quite aligned in its (pre-soldered?) clamp. I’d suggest pushing it upwards so it’s fully enclosed.
The right-hand sensor looks to be on a bend? That seems less than ideal, but not sure what could be done about it.
At a minimum I would recommend wrapping both sensors in a good layer of insulation. For best results, take them out and apply thermal compound before refitting, though I appreciate it’s a tight space to work in.
Plotting flow and tank temperatures on the same graph will make it easier to compare.
Also, look at the period when the compressor is off but the primary pump is running. The flow and return temperatures should be the same while they drop.