The lug is sometimes less than well fitted to the bbl contour. The sweat solder attachment process lacks strength and soft solder is a very poor filler material.
The J-Spring type forend attachments put a lot of forward tension on the lug when snapped into place. They keep the bbl pushed forward nicely, but at the expense of pushing hard against that lug to do that.
The latch type can pull the lug out if the fit is not correct. Too awful tight where the latch doesn't close and needs a slap to close is just pulling on the lug to close the forend tight to the bbl pulls on the forend lug..
..and sometimes the old solder joint just gives way. 100yr old soft solder joints that are under some tension can do that.
The reinforced lugs with their extended & sometimes wider short ribs to the rear help with greater contact for soldering.
But fit is still the primary need as a sweat solder joint is useless w/o it. A thick layer of soft solder betw the two surfaces has little strength.
Here's some pics of a couple lug failures. You can see the poor solder contact betw bbl and lug on the one.
The lug(s) get loose and move forward pushing on the bottom rib loosening it and crumpling it
at the same time.
So straightening the rib and re-solder of it is usually needed.
Careful cleaning of the surfaces is needed. I used a simple scraper most of the time. The old solder and flux shaves off easily as does the soft steel the parts are made of.
The bbl surfaces clean up easily with scrapers as well. Much easier to manipulate than files IMO.