Hello! Welcome to the forum.
Was the oil leak in the same area of where the oil filter gasket was? If so, I'd assume it has to do with the work previously done. Figure 1 in our tech article for the gasket replacement shows you the area of where your oil filter housing is located:
BMW E90 Oil Filter Housing Gasket Replacement | E91, E92, E93 | Pelican Parts DIY Maintenance Article
Unless you inspected where the oil leak was coming from, it's hard to say exactly what was leaking, but since there were no leaks previously when you brought it in, I'd say it's safe to assume it may be related to the work done.