The basic flow when thanking someone on a mobile page (Firefox on Android, if that matters):
Tap the "Thank" link, get brought to a new page with a "You sure?", tap "yup", get brought to a "good job!" ending page, with no link back to where you were.
(Phrasing may be different ;) )
Maybe this is just Thanking someone for a Flow comment? On an edit history page on desktop it all happens in situ. But I just thanked a flow comment and it took me to a dead-end page on desktop as well (eg: Special:Thanks/Flow/gobeldigook)
Ok, after trying to reproduce in front of Roan it worked on desktop (I probably had some weird JS loading error).
I'm invalidating this bug, I was confused on cause at the beginning and now it's messy. I'll file one about the dead-ended-ness when javascriptless doing a thanks.