Hi AcGolfer.
All foam materials will both soften (virtual impressions) and/or develop visible impressions to some degree over time and won’t stay perfectly flat but if they are higher quality and more durable materials such as latex they will be significantly less and take longer to develop than lower quality and less durable materials and any minimal softening or impressions won’t generally affect the comfort and support of the mattress.
Latex, in general, is the most durable foam material and generally holds up very well for a very long period of time so outside of any defects in the latex itself (which is very uncommon) if there is any sagging it would be in the quilting layers or the base layer under the mattress or in many cases what they are calling “sagging” is really what I call “virtual impressions” which are the result of choosing a comfort level that is too soft.
There are several possibilities that can account for your experience that are listed in post #4 here.
As you are mentioning a 1 ½ " deep impression and since is blended Talalay latex in your Adjustable Ultra Plush is among the most durable foam materials it will soften much more slowly than any other type of foam … so foam softening or breakdown is rarely an issue with latex unless it was defective or “on the edge” of being too soft for your body type and BMI when it was new (softer foam is always less durable than firmer foam regardless of the type of foam). In these cases, the issue is not about the quality or durability of the materials but can be an issue of “on the edge” initial comfort choice or an inappropriate support system underneath the mattress … so I am glad that you reached out to Ken to discuss your situation and help you pinpoint any possible causes.
I would also check to make sure that your support system is still perfectly flat and that there are no parts that are sagging or that are bending under the weight of the mattress including the people sleeping on it. It should provide similar support to having your mattress on the floor and you can test this by putting your mattress on the floor to see if it makes any difference. If it does then it’s more likely that your support system part of the problem.
Generally, I do not usually recommend zoned systems if it is not done under the careful guidance of a manufacturer that has the ability to assess and customize any zoning to your needs, sleeping positions and body profile. Generally zoning by itself can create more problems than it solves, but it can certainly be useful and well worth considering for people that have more difficulty finding a mattress with the right “balance” between comfort/pressure relief (under the shoulders especially) and support/alignment (under the hips/pelvis especially) or who have more challenging circumstances or sensitivities, body types that are more difficult to “match” to a mattress, more complex medical issues, or who have a history of having more difficulty in finding a mattress that works well for them. There is more about zoning in this article and in post #11 here and the additional posts it links to but the only way to know whether any specific mattress (zoned or otherwise) will be a good match for you in terms of comfort, firmness, and PPP will be based on careful testing or your own personal experience.
There are hundreds of zoned mattresses in the market in many different mattress categories (including “all latex” mattresses) most of which use “fixed” zones but there are a few that use customizable zones as well. Some of the ones that use customizable zones include Flobeds which you mentioned and the Reverie Dream Mattresses (although they aren’t “zoned” in the more traditional sense of the word because they have cylinders that can be rearranged to provide different levels of firmness and feel).
As far as adding extra foam strips under the “depression” area while it can be useful in certain extreme situations … this can also over time can result in some odd body impressions and as a general rule, it is not something I normally would recommend, especially not before diagnosing the issue and finding the root cause of a 1 and ½ deep impression in an 8" mattress. If this is something you want to experiment with, keep in mind that the closer to the sleeping surface the strip is placed the more impact it will have and that due to the height and firmness differential, it can also create other issues such as unwanted “ridging”.
Phoenix