Jfredd's post in Modeling/Retopo lasso selection problem. was marked as the answer
Evil_Weasel (Discord) passed along workarounds for this, in case others are experiencing the same problem.
Create a new, empty UV set and switch to it in the tool bar, then the select tool behaves normally
or
Manually de-select the UV island in the UV Preview after selecting the vertices.
Neither is the ideal/straightforward way, but it gets us through.
It doesn't overshadow my thankfulness to the 3d-coat team for such great software
