Sort filter and ratings ( and taxonomies )

Hi, Suspect this might be a bug but asking first in case I’m doing things wrong.

Using 1.9.7

I have a loop displaying some CPTs which all have categories attached and a custom field for Rating.
The Rating field is an ACPT one ( the whole CPT is ACPT based ). When displayed this shows a value such as 5/5 or 3.5/5.
I have Admin Column and added the Ratings field to the CPT list. This shows different values in a 1-10 range.

I have a Select Filter and added two Ratings Sorts ( One Ascending One Descending ) So users could sort by rating.

When used the sort does not respect the actual assigned values. One in particular always appears out of sequence.

I also created another sort field based on a numeric field ( year ) and this sort works perfectly even with pagination.

So I suspect that the ACPT Rating field may be the culprit, but not sure how to fix.

In a related matter - How do I sort by a Category? In the Academy it says the Select filter should allow sorting by Taxonomy but I don’t see any option for this in the drop down.

Cheers