Hi rekstudio,
There are too many unknowns, variables, and complexities involved to “diagnose” mattress issues with any degree of accuracy on a forum without being able to feel what you feel or see you on a mattress (it would be like a doctor trying to diagnose medical issues without being able to see or examine a patient or even talk with them in person) and this is especially true when I don’t have a frame of reference for mattresses that you have tested or slept on previously that have worked well for you. I also don’t know the thickness of the latex layer in the mattress or the type of latex (although I’m assuming that it’s probably Talalay).
In general … now that you have a better frame of reference for a mattress that isn’t working well for you … I would use your very careful and objective testing (using the testing guidelines in the tutorial post) which would be the most effective way to decide on a mattress that would work better for you.
Having said that … while there could be some “non mattress” issues involved such as a foundation or bedframe or a mattress pad or protector you are using (see post #2 here) … if I had to make a guess based on your description it seems to me that your comfort layers may be too thick or soft and that you may do better with a firmer mattress.
This could be just a normal part of the mattress breaking in because the middle of the mattress isn’t generally used as much or it could be from the support system under the mattress. In some cases if there is some flex in the support system on each side of your mattress the middle may be firmer if there are two twin XL foundations under your mattress where the firmer side supports of each twin XL foundation come together in the middle and in some cases this can be felt “through” the mattress. The best way to test this is to sleep for a few nights with the mattress on the floor to see if it makes any difference.
Phoenix