The Model 21 forend lug and ribs are occasionally known to become loose and need to be resoldered. Shooting the gun with a loose forend lug can cause other damage like kinked lower ribs. Give your forend lug the wiggle test.
Occasionally, like several of them failing at any given Vintagers get together. 4140 is a difficult steel to solder, and there existed a well documented design flaw in the fore end lug attachment, which was compounded by the later beavertail. Repair of this is, well, spendy.
That said, the old double gun habit is spendy. All of them need repairs sooner or later.
There are 21s that have never failed. There are lots that have. You might want to have that looked into, by a pro.
Best,
Ted
