I started noticing this after updating to PB 5. Regardless of what tool I have selected before entering Play Mode, upon entering Play Mode the tool always switches to the PB Move tool, and switches it to Face selection.
Even if I were in vertex selection mode prior to entering Play Mode, it will change to Face Selection upon entering Play Mode.
Note that this is occurring on entering Play Mode, not leaving Play Mode. If I enter Play Mode, then Pause, then choose the Vertex selection tool, then leave Play Mode, the vertex selection tool will remain selected (as it should). It appears to be something specifically about entering Play Mode.