I just noticed this error before upgrading to latest version. Upgrade did not solve it.
When I select 6 out of 10 photos in a PM Contact Sheet I see the correct number of photos in the bottom left counter.
When I reverse the selection with CMD+O that swaps selection/non selection the counter in the bottom left doesn't get updated the first time still showing 6 selected instead of 4 that is actually shown selected.
Pressing CMD+O again swaps both the selection and the displayed counter, but they are out of sync - so it keeps showing the wrong number whenever swapping the selection.
I'm using Photo Mechanic Version 6.0, build 5260 (0666e52) with OSX Mojave with 10.14.6 (18G103).