I don’t really have an answer for you question only speculations or theories I can’t back up with hard evidence.
I don’t like using hard resin for potting. It makes virtually impossible to do any sort of repair work, fault finding, repairing or things like that on the product, as you have found out. And to it also looks like resin shrinks a tiny fraction (using my completely unscientific eyeball measuring) when it have cooled down this might just come down to my perception and it’s not something we have actually measured as we haven’t had the need to do so, but if in fact that’s what it does, it could induce tension on components and the PCB
So what we sometime use when we need to encapsulate electronics to keep out moisture or mechanically stabilize components are magic-gel and magic-rubber from raytech (RS-online nr. 703-1231 and 703-1228). They do they trick for us, and we can actually get to the electronics if we need too.
Mind you, I work at a marine research faculty at a university here in Denmark, so I never work with bulk production, only prototypes, one-off’s and very low volume products for the researchers and scientists. So those sorts of products might not be viable in your case.
Only place where we use hard resin (3M scotchcast 2131) is for submersible applications with connectors and cables.