Where are the pics of the bubba? Have any pics of the dead leaves? I have a Cannibis Care Manual I use for identifying plant issues. I scanned through it but didn't see anything on cola specific except fungus related. Do you have enough air flow at the top of the canopy? You might have Nitrogen deficiency by now depending on how you are feeding the plant. What's your feed schedule?
Here's the link.
https://www.rollitup.org/newbie-central/651754-bubba-kush-26-days-since-4.html
I'm feeding Fox Farms Big Bloom and Tiger Bloom. Feed about once a week. Feed let it dry then water with Ph'd water let it dry then feed nutes again.
Not sure why it;s isolated to the cola's top leaves. I have two fans working air flow, I let my fans run during lights out also. It is just the top set of leaves no ther leaves on the cola is affected.
Swelling would be expected. What's the breeder's photoperiod for the bubba (how many days of flowering to mature).