I agree that a butt joint on thin copper is the worst choice using soft solder, but a good joint will last, witness many old flasks.
When I’ve had to repair flasks that were coming unstuck I always opted for pulling them completely apart [unless they’re leather covered, then it’s epoxy glue and hope].
My method is to clean up both halves, remove all dings and dents, then put a sheet of new emery paper on my old surface plate and gently rub both faces flat, test until they touch all around.
Emery the inside clean for at least 1/4 inch then tin with 50/50 solder.
Check the fit again then hold them together with string or rubber bands in three places.
Using a Soldering Iron introduce heat and some solder.
If all goes well the joint will run together as the iron advances and you end up with a continuous and strong enough job.
NEVER use a gas torch, stick with the old time way.
Having said that I have an original Hawksley iron [steel?] flask that was electrically welded in manufacture.