Hi John,
I'm not very familiar with subframe focusing as I don't use it. But I did just take a look at the code. The Autoselect star functionality basically works on HFR (rather than brightness - although usually the brighter the star the larger the HFR).
If star detection method = SEP...
1. Look at all stars except those within 10% of the border of the sensor.
2. Order the stars by HFR.
2. Go 5% down the list (from largest HFR).
If star detection method is not equal to SEP...
1. Picks the star with the largest HFR.
So you could try a star detection method other than SEP and see if that helps.
Another thing you could try is selecting a star before running Autofocus as there is code that looks like it tries to maintain the same star detection. (I've not tried this so I'm not sure it will work).
So these might help a bit.
If you're interested we could see if I can help optimise your setup for using full field. With your SCT at 2000mm FL the issue I imaging you're having is detecting enough stars for the statistical averaging to work effectively. I did a video on optimising star detection that you might find helpful:
Alternatively if you send me the focus frames and log from a full field autofocus run I can take a look and see if I can improve parameter settings.
HTH