According to GuildStats' data and my own analysis, you are correct. Ferumbras on Garnera was supposed to spawn between July 06 and July 20, but it didn't. As you said, it's probably because Ferumbras was set to spawn on a day in which a second Server Save happened before Ferumbras spawned, making the server believe it spawned already.
These extra server saves have all sorts effects. It's important to note that a second server save is different from a reset, as when there is a reset the data is restored to the last point, while on a Server Save everything that happened is saved and kept. These extra server saves change the Dream Scar boss a second time, they skip World Change stages (such as deepling), they move Rashid (if it's past midnight CET/CEST) and they can also prevent triggered bosses to spawn.
For all we know about the raid algorithm, bosses are set to spawn when a Server Save happens. It's probably that at this moment the time it will spawn is also selected. Being triggered to spawn seems to be enough to register that boss as spawned and reset its cooldown. This for example prevents a boss for respawning if it isn't killed. As much as I can't prove it, I remember seen this happen before. You can try to email Support about it, but if I recall correctly last time they weren't willing to do any manual changes to help.