After a period of cloud I have now been able to collect more evidence of what is happening.

Please note that whenever I manually select the star to use for focusing, it works correctly, it is only ever an issue when using autoselect.

There seems to be two main circumstances -
1) It selects a star and centres it but then uses another star in the subframe to focus on. This sometimes involves two stars close together where it jumps back and forward between them.
2) It selects a star but instead of being centred in the subframe it is near the edge. This sometimes work fine but sometimes slips outside the frame while focusing.

The three examples attached
1) Selected two stars close together - nothing apparent in the center
2) Right on the edge
3) Plenty of stars in full frame but nothing in subframe
4) Star in centre but using one near the edge