Hi Jesse,
If you tested the mattress on an adjustable bed (which has a firm non flexing surface) and then you put your mattress on a box spring (which flexes as opposed to a rigid foundation) … then it will certainly have an effect on the feel and performance of the mattress and could easily allow your heavier parts to sink in more (which isn’t a great idea for a stomach sleeper as you know). This would be especially true with thinner mattresses.
You could easily validate this by putting your mattress on the floor and seeing the difference. I would just make sure that there was a blanket or something on the floor to protect the mattress from getting stained.
If this is the case … then exchanging the more flexible foundation for a flexible one would likely solve the problem (it wouldn’t require a change in mattress).
While it’s true that subjective memory can change very quickly and also that there is an adjustment period with a new mattress … there would definitely be a difference if a mattress was used on a flexible boxspring vs a solid foundation and if you tested it with a solid platform then that’s probably the best choice at home as well.
Most retailers would have a foundation that was rigid and non flexing and had an appropriate amount of slats in it (no more than a 3" gap between the slats). There are also some very good choices in the foundation thread here if they don’t offer one that is appropriate (which would surprise me).
So I would put the mattress on a solid surface tonight (like the floor) and see how things go.
Phoenix