Bacteria strains can and do "drop" plasmids unless they are under constant selective pressure. Are you culturing with the correct antibiotics?

Mutations are very rare, even in bacteria. My best guess is your bacteria just lost the plasmid due to incorrect culture conditions.

Thank you for your answer. I'll pay special attention to this question when I experiment again.

I agree with Katie A S Burnette that mutations are normally pretty rare. However if the protein being expressed is somewhat toxic to the cell then you may have a fairly strong selection for mutants that fail to express. This might include a variety of deletions in the plasmid or other rearrangements in addition to point mutations. It really would depend upon the product.