On Android, if I'm in a lesson and go to Vocabulary > New Words, if I mark a word as known, it does not save. You can see this by marking lots of words as known, close the pop-up, close the lesson, re-open the lesson, then Vocabulary > New Words. You will see all the words you previously marked as known are still in the list of New Words and if you check your known word count/statistics, nothing has changed at all.

It appears I can add new lingQs on Android and they save, but I cannot add/save known words.

I can mark words as known on the browser and it works fine.

EDIT: The reason why I prefer to go through the Vocabulary > New Words on Android instead of the browser is because, firstly, it gives you the option to mark it as a lingQ, which isn't available on the browser, and, secondly, it gives a 'preview' of the top definition (which is important because if I'm importing auto-subtitles the spelling is sometimes wrong, so if I see it has a manually-created community definition, it means that it's spelt correctly or at least I can open up the community definitions to check, which is not available on the browser, as mentioned).