I suggest you read it properly, what the OP actually wrote.
He clearly stated the pressure increased on cooling / on the heating going off and without him needing to top it up. I even asked the OP the clarify that point and he confirmed it. The OP also said he renovated his flat, which suggests he owns and is responsible for it.
It’s nothing an annual service won’t fix. Vaillant are renowned for this and I dare say the expansion vessel runs on its limit.
Small temperature changes can help bring the boiler back on. As well as making it go off again.
It doesn't explain how the boiler can fail on an F22 due to low pressure, then mysteriously top up its own pressure to be able to function again. Heat input causes pressure to rise, when it cools pressure reduces. The complaint seems to be that it is doing the precise opposite of that - despite the protests to the contrary.
What I believe has happened, the boiler pressure has dropped to 0.5 bar, on that model of pro once the pump kicks in (the 2nd component to be energised), the pressure goes up by 0.5 bar, so could have brought the heating on that way.
A temperature rise in the property heats the water in the system which expands and raises system pressure enough to bring the boiler back on. Especially when ev is depleted.
Then it is under Vaillant warranty so get them out, I agree probably an expansion problem, which they would say should be checked at every service, but as it isnt 12 months old yet, that is not a get out they can use, as @jeff the gasman says the expansion vessels on these are very poor
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local