The moldy odor which started in the 1st atmospheric river of 2022 continued to 2024 despite changing filters & drier weather.  Higher humidity enhanced it.  It became quite overwhelming & there are theories it could cause respiration problems.

The leading theory is it started with vitamin C megadosing.  The ascorbic acid caused the sheets & the clothing to smell like musty paper.  That might be getting captured by the filter.

The internet says ascorbic acid smells like hot dogs.  They might be thinking of formaldehyde, but few animals now online have any memory of formaldehyde since it was outlawed after generation X.

Another theory is the brand of filter from 2022 contains some impurity.  It was always assumed the Hunter 30055's filters went out of production & the current ones were all old stock.

There could be impurities in the blower itself, but this wouldn't fit the pattern of a fabric smell enhanced by high humidity. 

1 solution is to spray the filter, but this would be temporary & clog it.