Increase humidity with HRV?
Ah, I think I’ve found the answer:

Admitting cold outdoor air into a space will lower the indoor RH. When the lower temperature outdoor air is brought indoors and heated, it loses moisture and reduces the overall RH.
For example: if the outdoor temperature is 0°F and 50% RH and the air comes indoors and is heated to 70°F, the residual moisture after heating the outdoor air will only be about 3% RH. Even on a nice, sunny 35°F and 50% RH day, the residual indoor RH will only be about 14%.

So by heating the cold, high-RH outdoor air up, the RH content drops tremendously. By introducing this extremely low-RH air into the house, you can expect the indoor RH to plummet.