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.
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.