Never hesitate to disagree. I'm a compliance person, not an attorney and my opinions are not always correct.
There would be no reason to assign the mortgage. If the note is assigned that note is secured by the mortgage and you have the right to exercise your security agreement if the note goes into default and as you said that security agreement gives you, or it should, the right to start receiving payments from the assigned note to offset your debt secured by the note and then by virtue of that note you have the right to exercise actions against the mortgage should your assigned note go into default. So your loan is secured by a note not the mortgage.
If the mortgage is assigned to you as security for your loan then your loan is secured by that security agreement (mortgage). The mortgage now secures the original note plus your note. IMO, that makes your loan now dwelling secured because it is not only secured by an assigned note but is also secured by an assigned mortgage. You are now the mortgage holder.
I'm sure there would be some legal hoops to jump through if you had to exercise your default remedies against the mortgage but I still think your note is dwelling secured by assignment of the mortgage.
Now with all that said, it's not really an issue I would push to great lengths. I'm just throwing out another opinion that should be considered in deciding whether to report the loan or not.
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The opinions expressed are mine and they are not to be taken as legal advice.