I would bag and remove, maybe do an autopsy on the flower to make sure no critter accidentally got in there and infected it. If none then your environmental conditions need to be changed as it’s then obvious you have good conditions to produce bud rot. More air, peroxide spray, dehumidifier etc may all be necessary.
Yup, definitely budrot. As 410 said you need to increase your air movement. Half of north America has nights outside in 50s-60s and super high humidity 90%+. Just because your indoors isn’t, doesn’t mean the germinating spore that absolutely love these conditions can’t get into your home. That’s why I modified most of my intake ports to have filters.