Yeah unfortunately that is bud rot. I would add another oscillating fan for starters. Then I would find a way to keep all your fans on 24/7, you want stale air removed and replaced with fresh air, To keep good co2 levels lights on and good oxygen levels lights off, stopping stale air entirely which will help toward stopping this problem happening again. The extra oscillating fan will also help, you want as much air flow over the plants as possible this will strengthen them and stop pockets of stale air forming mid canopy.
I assume you don't use a carbon filter with you previously not using an extraction fan, but they are handy as stopping light leaks. Otherwise you could attach some ducting to either end of the extraction fan as well as your bends to try and block out light. Obviously you don't want light leaks. There are ways as simple as a cardboard box to block light without sacrificing to much airflow or causing to much resistance to the fans.
Possibly called a light trap or light baffle, not 100% on the name. But basically a box with different sections all painted matt black inside
Anyway, sorry to hear about the rot, I had it happen to me once and its an horrible thing when all that hard work gets lost to rot.
I hope you find ways to make sure you never see this happen again.
Good Luck!