First a little background. I played "tenor" bone professionally back in the early 70's. I wasn't great but earned a living. Never had this problem back then.

Ruptured my chops back then and took 40+ years off.

3 years ago I started playing again and switched to bass bone. Much less stress on the chops and no high notes.

Now if I have an intense practice session or a long gig I get a split lip. Always in the exact same spot!

What's going on and how do I break this cycle?

Drink water, don't stretch your lip across the length of your teeth, and eat citrus fruits in case it's scurvy. Old wounds opening up is a dead giveaway.