I made the spring/fall comment thinking about critters but yes I'm 10 lbs heavy right now. Longer nights means more blueberry cobbler time for us humans
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Your observations are on the money but the structure of sapwood and heartwood is the same, the cells do not change, they die and are then filled with extractives, those compounds I was rattling off earlier. They make the wood less permeable. In pressure treatment heartwood doesn't get treated, the liquid won't penetrate. Foundation grade treated wood specifies all sapwood for this reason, it can be treated through and through.
The roots, or hyphae, of bluestain fungi go through the cell to cell pits and punch through cell walls making them more permeable as well. Ever notice how much faster blue pine wets out than unstained wood? Blue pine gave us fits in the staining of furniture, it would absorb stain much better than nonblued wood. Bluestain doesn't affect heartwood since there is very little sugar within the lumens there.
If you look at wood design values or span tables it is not broken up as sapwood and heartwood because there is no structural difference in the two. The cells don't change as much as they are simply backfilled with tree poop. There is a practical exception to that for compression in woods with high extractive content but it is not used.
Lots of things enjoy the high sugar/starch layer just under the bark. It's one reason to get the bark off as soon as possible and not to leave any on after sawing. Bark provides hiding places, preserves higher moisture and gives ready access to that food layer.
You can probably see where I make observations and then try to check them to see if they are correct. I made a guess about ponding washing out starch and sugar and I've been wondering how to check it. Looking for research would be the first step. If there is nothing then home experimentation would be a start. Starch would be easy, a sample that was cut in half, one half ponded, one half on shore, dry both, an iodine wash stains starch so should make that obvious. The only way I can think of to check for sugar would be a refractometer like you check fruit sugar content with, you would have to make a slurry of the wood after the treatment. They aren't too expensive, not sure if it would work though. There are probably other ways.
Myth seeks to explain, science seeks to know. By figuring out how to check it out we move from anecdote, which is only slightly more than myth, to true knowledge, that's when I no longer have to say "I don't know". It might sound kinda stuffy but that's when I know I am really in tune with what's going on. From there I can build on a solid foundation.
