In America that is more usually true. Here in the UK it is not usually true unless the problem is left to run rampant. Condensation, breathing, simply living, can be the root cause of black spot mould in all types of housing stock. Most people have small signs of it in their homes, whether in the corners f window frames or by outside doors, etc. This is perfectly normal in many houses and lifestyle, just being, is often a major contribution. there are two things:
Tenants: must use the property in a 'tenant like manner' and that includes them taking normal precautions against condensation mould
Landlords: must also take precautions to protect both the tenants and the property.
I usually tell tenants to buy a good mould spray, like HG, and to use it at the first sign off black spot mould. That is their responsibility. It is also down to them to tell the landlord if the mould recurs
At which point the LL has to effectively assess the issue. Is it the tenants actions, or lack of? Is it an issue with the house and if so how can it be remedied?
@LadyMaine you could (probably should) buy them some good mould spray and a good dehumidifier - like a Meaco 20l one. Show you are taking it seriously and want to help them remedy the situation. Ask them to get back to you after they gave cleaned and used the dehumidifier, cost of running one below
I've also included a fairly even handed guide to damp and mould that gives a lot of advice.
You should also acquaint yourself with the Fitness For Human Habitation Act, also attached. I find many LLs are unaware of it and can get caught out on daft things
You could (probably should) get an expert in to tell both you and your tenant what the issue is and how to remedy it. An independent voice would, agan, show you are taking this seriously and give you some protection should things deteriorate due to the tenant's choices. You'll have to pay but it could be cheaper than a S21 and reletting
blog.meaco.com/cost-to-run-dehumidifier/
www.permagard.co.uk/advice/damp-rental-properties-what-to-do
www.legislation.gov.uk/ukpga/2018/34/introduction/enacted