We diagnosed the issue as a problem with the last step of the TA upload process - associating the placeholder with the data. It seems like this cannot always happen immediately after the data upload has completed. Tailored audiences that fail this association step show up as processing in Twitter Audience Manager but in fact will never complete processing since the association between the placeholder and the data will never happen so the actual match processing part never starts.
Don’t know why this is happening on V0, but the status in the Audience Manager is obviously incorrect. I haven’t seen the Audiences change status yet from Processing to Too Small - they were uploaded over the past few days, but I suspect that’s just another false error message like the original Processing message.
It’s unknown if Twitter API has implemented something new - caching, script or if it’s just that the deprecated API is now overloaded as they try to move resources over to V1, but this is definitely a change in behaviour. from last year.