The filter goes onto a adapter plate for the regular filter mounts... that is the silver/gray circular thing that you seen when the filter is off. This is your most likely problem area.
- There is a gasket between that plate and the block. The gasket can leak.
- Or the adapter can get bent/distorted if/when removed
- Or the threaded pipe adapter in the center, with the hex machined into it that is used to tighten the adapter plate in place, can be loose.
- And IIRC the later Magnum filter adapter is shallower than the LA one so may seal well against an LA block
