Paul Tarlow

Difficult to know for sure, but sounds like a slumping mold.

Deep square and rectangular molds will almost always end up with curved edges that dip in towards the center. Slower slumping will minimize this. You can probably get to straight edges if the flat piece has sides that are bowed out – but it would require testing.

You can always perform the anneal-hold at a higher than required temperature so I would continue to hold at the prescribed temperature and maybe extend the anneal cooling a bit lower.