Ventilation / airing in the passive house - duct odor / musty laundry

Hello everyone,
we are really at our wits' end and are looking for competent advice regarding our wastewater problem:
We live in a passive house. Our washing machine (new) is located in the attic. It was therefore connected at the highest point of the house – the pros and cons can be debated, but let’s just assume there was no other option to connect the machine in this room.
For quite some time now, we have had the problem that the laundry sometimes smells very musty, especially when washing at temperatures above 40 degrees. I installed a siphon before the wastewater connection and raised the final connection of the drain hose to the upper edge of the washing machine. This should provide enough “water seal” to prevent the siphon from being emptied.
Since we had no ventilation of the wastewater pipe, a sanitary professional also installed an air admittance valve on the same connection. Although this has helped improve the situation, we still have sewer odors in the bathroom and the laundry still smells musty. Therefore, we cannot use washing temperatures above 40 degrees. We find this not only unhygienic – in our opinion, it just cannot be like this.
Background: The lower part of the house is separated as a granny flat from the ground floor and first floor, where we live. The floor plan of the ground floor does not provide a connection for the washing machine. So, we are dependent on the space in the first floor and this connection.
Good advice is now expensive. Does anyone have any suggestions?

Hmm, musty smell of the laundry usually does not come from the drain but from the insides of the washing machine. In other words, I would rather look for the cause there.

Our washing machine is also connected at the highest point of the house: in the [OG] at eye level. There is nothing musty there!

but it is strange that it smells worse after a hot wash than after a cold one?

We also initially blamed the washing machine - but a brand new washing machine of the same brand has the same problem. As already described, the peculiarity probably lies in the fact that we have no external wastewater pipes – and therefore no external ventilation. We only have this ugly stub pipe installed in the bathroom, and that is the only one in the entire house. To help visualize it better, I took a photo... see attachment.

We have now had the problem for a long time that the laundry sometimes smells very musty, especially when washing temperatures of more than 40 degrees are used.

The laundry from the machine or laundry in the room?

And the thing with the temperature is also not quite clear to me; after all, it shouldn't change the amount of water much.

Is there a bathtub in the bathroom?