In a regular fridge, there usually are two air vents between the fridge and the freezer.
I'm just going by the Vitrifrigo DP150, which is the slightly smaller sibling to the DP2600 the OP has. Both have a separate fridge and freezer compartment (two separate doors, not one inside the other). There is one compressor and one mechanical thermostat located in the refrigerator.
In the DP150 there is no air transfer between the freezer and the fridge (to speak of, not saying it's hermetically sealed). There is a cold plate in the freezer and also in the refrigerator on the back wall (the type that is painted white with "veins" running through it).
On my buddy's DP150 he had this vexing problem where as the weather got hotter (we are talking extremely hot here; Arizona in summer, living space at ambient temp) the freezer got colder as the refrigerator got warmer.
So where before, let's say you had the thermostat dial on 75%, the freezer might be at 12ºF and the refrigerator at 38ºF. (Even at its best it was a balancing act to keep ice cream frozen and produce not frozen, but it did work out just fine.)
When it started malfunctioning, the freezer at the same setting would be at say, 4ºF and the refrigerator at 48ºF. At it's peak of badness, the freezer would be at around -12ºF and the refrigerator at 55ºF. (By then it was at full setting on the thermostat, but at any rate the concept was the same.)
When the refrigerator was so warm (and the freezer SO cold), there was almost no light frost on the cold plate in the refrigerator section (but there should be; not a ton, just a light film). DC wiring was robust and anyway it was the same when plugged directly into household AC (this control unit was AC/DC) and bypassing everything else. No error flashes.
We installed the refrigerator ourselves and had added around 2" of extruded polystyrene to the top, sides and back, and 3" to the top. There was a vast amount of ventilation because (this was in an RV) the cutout used to house a propane refrigerator, and so there was a huge air vent down low on the outside of the RV (just outside the compressor), then a chimney up to a roof vent (this is how propane refrigerators are installed, because there is a fire burning in the back, essentially).
We changed the resistor on the compressor to make it run at the full 3,500 RPM. No real change.
When the weather cooled down, it went back to normal. (This was all happening during the first season of Covid, so between that and being in Arizona --plus the owner's tendency to ignore things -- no marine refrigeration techs called in).
This summer, back to the same problem, but only in extreme heat.
(This whole time the RV was in storage under a roof, but the refrigerator was still being used as spare food storage.)
So this problem sounds very similar, and I'm guessing the DP2600 is set up similarly, being the next model up in the size range (i.e two separate doors, one thermostat and compressor). As I mentioned up above, it was always a slight balancing act to keep ice cream hard but not freeze the fridge, but workable. It was only when it got very hot that this weird spread occurred. Freezer got super super cold; fridge got warm.
We never totally solved it (I might have pushed harder by calling techs, but it wasn't my refrigerator and the owner is a bit more laissez-faire). We only know what it seemed not to be (not the electrical supply; not poor installation, not anything that throws an error code), and that when the weather cooled down it went right back to normal (so nothing "broken" that precluded normal operation later on). We both have other Vitrifrigo refrigerators with Danfoss compressors so are used to how they work (usually).
OP: Sorry if this seemed to be turning into a story of my buddy's refrigerator. I only elaborated because the problem sounds SO similar to yours (except you are not in Arizona), and as far as I know the DP2600 is very similar to the DP150 but a bit larger.
Here is a photo of the DP2600. As you can see by the notch, the compressor is at the bottom of the refrigerator section.
I can see the cold plate at the rear of the freezer section. I can't be certain, but it looks like on this model the cold plate for the refrigerator might be on the "roof" of the refrigerator vs. on the back wall like the DP150. Otherwise they look very similar.
OP: Is there any light frost on that plate in the refrigerator? I'm not talking big honking ice dams (obviously that's not good), but merely a light frost that should be there when it is working.