@EA_AticThe mouse I had the mapping problem was the Logitech G700s. Only the lower thumb buttons (G4 and G5) were recognized. The other two (G6 and G7) as well as the rest of the buttons peppered around the mouse (G8 - G11) could only be used in games by mapping them to keyboard events.
My current G305 only has two thumb buttons and I've had no desire to map the one extra button - I've felt being able to confirm that battery has plenty of power is more important.
Edit: Also note that these two mice are from generations apart, and the mouse software supporting them is different: G Hub for the recent models and LGS (Logitech Gaming Software) for those from a few years back. (SetPoint for the really old ones from late 2010's.)
Edit 2: For some reason, editing this post on mobile kept applying the spoiler tag on the edit. Couldn't remove it.