ᐅ Increase in pressure in cold water line during hot water heating-up

Hello everyone,
we have been living in our house, built in 2006, for one year now. Since moving in, I and two heating specialists have been struggling with the following unresolved issue (please pay close attention to the distinction between hot water (HW) and cold water (CW)):

Regarding the installation:
On the ground floor: the cold water pipe enters the house → water meter → shut-off valve → pressure reducer, set to 3.5 bar (51 psi) → cold water pipe continues...
On the upper floor: heating room with gas boiler Viessmann Vitodens 200 and Vitocell hot water storage tank. The problem only concerns the hot water part, so I will describe this in more detail: The hot water tank has a non-return valve combined with a backflow preventer on the cold water supply line, as well as an 8 bar (116 psi) safety valve. Then, of course, the outgoing hot water pipe including a shut-off valve and a circulation line with (presumably defective backflow preventer—more on this later) as well as the circulation pump. There is no expansion vessel installed for the hot water part of the heating system. There is no separate pressure gauge on the hot water tank.

Regarding the problem:
Every time the hot water tank is heated, the pressure at the pressure reducer on the ground floor (i.e., on the cold water line) rises until the 8 bar (116 psi) safety valve on the cold water inlet of the hot water tank activates and discharges water. It is clear that water expands when heated and has to go somewhere, but it must not flow back into the cold water line. I should therefore not see this expansion (=pressure increase) at the pressure reducer on the cold water supply line on the ground floor.

Both heating specialists initially say the same thing: hot water is somehow pushing back into the cold water when heated. They also say there are theoretically only “two” points where hot water could flow into the cold water:
a) At the cold water inlet to the hot water tank
b) In any of the installed fittings
—> Otherwise, there is no direct contact point between cold water and hot water in the entire installation.

Previous, unfortunately unsuccessful, measures:
1. The non-return valve on the cold water supply to the tank was replaced. When this valve is closed, no hot water flows through it, so it closes properly. Nevertheless, the problem described above still occurs even when this valve is fully closed. Therefore, in my understanding, this point can be excluded as the cause.
2. I have replaced four fittings so far, starting with the oldest. These were three single-lever mixers, replaced by Hansgrohe single-lever mixers, as well as one exposed thermostat fitting in the shower, also replaced by Hansgrohe. Unfortunately, no improvement; the problem still occurs.

Further observations:
If I close both the cold water inlet and the hot water outlet from the tank, the problem still occurs! From this, I conclude that the backflow preventer on the circulation line must be defective, as this is the only remaining path that water can take. Unfortunately, I cannot shut off this line (yet). In principle, I think it does not matter, since there should be no connection between hot water and cold water here.

Now to my questions:
1. Would you agree with the hypothesis that only one fitting can allow hot water to flow into the cold water?
2. If yes, is there a way to locate the fitting responsible?
3. Am I overlooking something? Any misconceptions?

After a year of trial and error, I am very grateful for any advice.

Thank you in advance!