There are a whole bunch of things that could be going on. Are you married to stainless? Reheat cracking in heat affected zones of 300 series is a thing. This isn't my field, but stainless probably wouldn't be my first choice. If you reeeeeeally want stainless then 309 or 310 might be an option.

Have you tried any post weld heat treatment to relieve stresses in the heat affected zone? This might not work on some stainless. Again, not my field.

Which stainless are you using? Is the material very thick? Thinner might be better where huge temperature gradients are involved.