At the start of 2020 I booked a multi-city LHR-SIN, BKK-LHR all via CDG for end a August and start of September.
Out of curiosity I just searched for the flighs in the online AF timetable. The LHR-SIN gave the option to 'Buy a ticket'. But that option was not given for the BKK-LHR. Either the flights are fully booked (unlikely) or AF has already made schedule changes but will not notifiy me until two weeks before departure.
Has this happened at anyone else, whose flights are after the summer?
No cancellation of the unique booking left for the summer, yet. You should try to search for the individual legs (CDG-BKK-CDG and CDG-LHR-CDG) to better assess the situation.