Just to double check, you are booking on united.com?

Stale / phantom inventory is a common issue. AIt is a systemic issue that has been around since the begining of time. Inventory used by search engines is not always realtime (there is a fee for realtime access, and when a seach provides hundreds of options the airline / search engine don't want to paid for each request) plus carriers don't necessarily update their external inventory in realtime Between this issues, you can search, find a itin and then learn it is not available. Such as your first case.

Your second case looks more promising if TK show confirmed. But you still may need to push on UA before the 24 hour point to finish the booking.

Note this is not unique to TK-UA. But some airlines are slower to provide updates and TK seems to be in that category.

EF inventory is subject to the same issues. There is not public source that will not have these issues.