It may be an issue with the cleanliness of the socket in the hygro - they often can get a copper corrosion in there so using CRC and a cotton bud can clean it out and improve the contact. It seems, esp in temperate climates, that a protective anti moisture spray is useful every 6 months or so to most weather stations to protect the connections - and with any external cabled equipment, eg 1-Wire networks etc.
You may be right - it could well be in the hygro and nothing will fix it except a replacement. I have replaced mine twice due to the humidity sensor going south. Hence my issues with moisture…