See https://en.wikipedia.org/wiki/Special:Permalink/826603174#Thanks_not_working
When I tried to thank Davey2010 for his [[ https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&diff=826593042&oldid=826591709 | edit ]], it looked as though it went through. I hit refresh on the revision history, and it says I've thanked Begoon for their [[ https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&diff=826602345&oldid=826601573 | edit ]]. I tried a second time, and it thanked Begoon's other edit.
It seems it is thanking the most recent non-thanked revision on the page.
My [[ https://en.wikipedia.org/w/index.php?title=Special:Log&action=view&user=MusikAnimal&type=thanks | thanks log ]] also shows Begoon was thanked, and not Davey2010.
I'm using Chrome on Mac OS El Capitan, Vector skin.