Good question. It happened even with Frixions that never left my house (which is the majority of them) here in the northeast USA. 90F/33C+ days are not uncommon here in the summer, but the Frixion writing in my notebooks ever went "invisible", so I think we can rule out a heat issue.

The ink in most of the pens was still visible and hadn't gone "invisible", so I don't think it was working but laying down invisible ink. It felt like the balls in the ballpoints had locked up.