Any plumbers? Musty smell from new bath drain

[JackdawCrow]

We had a new bathroom installed nearly two years ago.

After about 6 weeks, we noticed a musty smell coming from the bathroom drain. Quite overpowering, as in the towels in the bathroom started smelling.

We called the plumber back who just said close the drain cover, which we have been doing. He also suggested that it might be a dry P trap, but I can see water in the drain when I look down it.

It is getting worse now and I am getting sick of having to remember to close the drain cover.

Has anyone experienced this, or got any solution? I have tried stuff like bleach and hot water etc. As I said, it happened after installation. We didn't use to get it with old bathroom.

We had this, it got worse and worse over around 2 years. Putting bleach etc down worked briefly but we eventually got the plumber back and he said it was fine but we insisted and someone more senior came
Turns out the trap thingy in the pipes was holding water in it which was getting smelly when the whole point of it was to keep waste water out.
He took the whole trap part out (I think) and its been fine since