On the Monitors, the welds cracking is reported to be a common problem. One which is semi-exacerbated by folks just having the welds redone.
As a common sense fix, my suggestion is a bit retro, but... if it works, then it ain't stupid.
Anyway, back in the day, when most racing
bicycles were made out of specialized, very thin walled, steel
tubing. The joints in the frames were fixed together by brazing/soldering the tubes into intricate (joining) lugs. Which served to spread the loads of the joints out over larger areas of the tubes, and to prevent any hard spots from being created in a frame's construction.
bicycle frame lugs - Bing Images
This gave the joints immense strength, but with little weight gain. And by preventing hard spots, the tubing making up the frames, remained dimple free, & thus maintained it's strength as well. AKA no "Beer Can" failures in lay Engineer-Speak.
When repairing something like a Monitor
, I'm not saying that one needs to go with full lugs on the joints. However, it'd make sense to weld some partial lugs over each joint. Especially those which will obviously be seeing the higher loadings.
Kind of like welding gussets onto say heavily loaded areas on a pulpit, or a stanchion base (in addition to the originally designed weld).
And I'm a big fan of both seam/lap welding things, in addition to slot & or plug
welding in addition to the 1st option. When it's viable. Call it a belt & braces approach.