Re: mask with deinterlacer...

Unfortunately, FieldsKit often request frames for the layer (where the plugin is applied) at other times than the current one. When plugins do that in After Effects (like FieldsKit Deinterlacer), the plugin does not get the frame with the mask applied first.

However, there is a solution: precomp your layer with the mask. And apply our deinterlacer to that COMP instead of to the layer directly. This will solve your problem.
Pete