Any potential issues having a 50mm butt at the seat post end of a seat tube?

Only 50mm of the seat post is supported and the lower part of your top tube/seat tube joint will be on the thin part of the tube. How big of an issue will depend on how far the seat post extends out of the frame. The potential for cracking at the ST/TT joint is probably minimized by the fact that this is a lugged frame. There still could be some seat post creaking issues with the minimal amount of support and the end of the post moving around in the butted area when the post flexes.

can you shorten the BB end? What tube is it, Spirit?

Columbus makes a spirit seat tube with a 70mm top butt, so they seem to think that is enough. I'm pretty sure they expect you to cut off the bottom. I suspect that 50mm will be enough without issues. I am not sure that DB seat tubes are worth it, the top butt certainly adds a little more uncertainty to extracting a stuck seat post.

One thing that I should have done on my SFL frame that had a stuck seatpost is to cut off the seat post to the length of the top butt. Seat posts nowadays are really long, and there is no need for that. BITD, I think they usually had 50mm minimum insertion, but nobody inserted them much more than that.

Trek has sold thousands of bikers with a significant diameter reduction at the top of the ST. I forget how much post it actually clamped but it's no more then the width of a hand. Being a small guy I have a lot of post in my frames even after chopping off a couple/three inches. Andy

The tubing will be Columbus SL. Seat post will be up out of the frame about 6 inches and will end inside the frame near the end of the butt. I think it will work OK as I am using a lug at the joint.