I’ve noticed lately that when removing members from a list through the API, the member’s tweets are still present in the Timeline. I have a list in this state as a test right now (0 members, yet many tweets):
When adding the member back to the list and then removing through the Twitter web app, the tweets are removed from the timeline. This does not appear to be a caching issue, as the tweets are still present in the list timeline several days later. It appears that new tweets do not accumulate in the timeline, but those tweets that were there in the timeline before the member was removed from the list remain there indefinitely.