We have created an enhancement to consider the separation of the Modify Keywords role permission into two permissions: one to allow the creation / editing / deletion of keywords and one to allow the selection and deselection of keywords.
The behavior we were using previously in GPOADmin 5.13.5 was considered unintended behavior and corrected as a defect in GPOADmin 5.14. Currently the only way to be able to select / deselect keywords is to have the Modify Keyword role permission which also allows the creation and deletion of keywords.
The enhancement to be considered for inclusion in a future version of GPOADmin is 489922