I’m sorry to say that it does look like bud rot. My advice would be to cut it out immediately. Remove the whole section and throw it away.
Monitor closely from here until harvest, once it has started it can spread quickly.
Try to keep them dry, rain and morning dew can make it worse/spread.
I use a flashlight and sit low to look up at the plant and check every bud, every day when I’m growing outside.
After harvest, take extra time to inspect all the buds, it can be devastating to miss some and have it develop in the jars.
Good luck to you!