I tested again on the app. It is happening when I click a yellow word and either make it known or change it to another level of yellow. The coint count numbers flash, but if I click back on the word it’s reverted back to its original level of yellow. Also if I change focus (minimize and bring the app back up again) it will have reverted back to it’s previous color.
As a separate test I tried moving a known word back to unknown. That appears to work correctly. Something has definitely changed and it’s been the last few days or so.
I’ve also tried on Chrome 77. It definitely is happening if I convert a word from yellow to known. It will flash to the new “color” and then within a second the color reverts back and the level is back to the old level. If I click it once more to known then it stays. I can then click it to unknown, known, etc. with it keeping the level each time. I have seen this when increasing the level of yellow (say from 3 to 4), but this seems to be much more inconsisent on whether it will revert. Possibly these are words I’ve already changed successfully before.