Your policy describes a residential property
A landlord policy is rated for residential occupancy. A tenant running a workshop, storing stock or receiving customers changes the risk, and a carrier learning about it after a loss may question the cover. The owner did not create the change and the owner holds the policy, which is the awkward part.
Customers at the property are a new liability
Members of the public coming and going is a materially different exposure from a household. If a visitor to a tenant’s business is hurt, the owner can be drawn into the claim, and a residential liability limit was not set with a public footfall in mind.
The lease is where this gets controlled
A clause requiring written consent for commercial use, and requiring the tenant to carry their own business liability cover, costs nothing and prevents most of this. Without it the owner is relying on finding out by chance.