Are PTFE stir bars resistant to all lab solvents?
This may not be the answer that you are looking for, but I remember a project, for which we had to replace PTFE stirring bars with their glass-coated counterparts.

It is true that PTFE is resistant to most of the solvents used, but it is not really hard and resistant to abrasion.

In a study where aqueous suspensions of metal silicides were stirred, we recognized very soon that the PTFE covers didn't last very long. The stirring bars were polished down to the metal core, which spoiled the experiment. Glass-covered stirring bars turned out to be a viable alternative here.