Not even AY knows yet. Aircraft are ordered years ahead and the airlines don't know what the world will be like when the planes are finally delivered. Their network planning has to live with the planes they have on a much shorter term process. If the business is good, they want to use all the available planes.

An extra A359 would cost a lot of money so with a small fleet it's probably cheaper to pay the EC261 compensations every now and then. As the fleet grows the number of tech incidents grows as well, so the business case for getting a spare becomes better.

If AY will at some point add these midnight departures to other destinations doable with a 24-hour rotation, they could pass the problem to other destinations as well so then AY99 wouldn't be hit that often. They already have BKK, but that's operated with an A333.