I buy my feed from TSC and have not had an issue with mold (I use Purina & Nutrina feeds), but our TSC keeps low quantities on hand and are sometimes out of what we feed. Broken bags are a different matter - especially on the Dumor alfalfa pellets, which always seem to have tape on them (I won’t buy them if they’ve got a tear, rip or have busted open).
I have, however, found a way to keep my feed from getting moldy/musty once I get it home in the high temps & humidity we have here in the summer. I use large plastic Behlen feed bins (link below). My feed room isn’t temp controlled (that would be so nice!). We don’t use nearly as much feed in summer and it would get musty smelling and I’d have to throw it out, wash the bin and start over. So, I went to Lowe’s and bought really thick, heavy duty foam weatherstripping with the adhesive back (the kind for an exterior door) and stuck it it all the way around the top lip of the bin to make it an air tight seal when the lid is closed & latched. No more wasted feed in summer! This also works on metal garbage cans (insulation on underside of lid).
https://www.behlencountry.com/product/multi-purpose-storage-container/