Apparently, the problem has been existing for years, judging by the internet posts. Monday investment portfolio DAY GAIN/LOSS figures are always totally wrong. Prices are correct, market values are correct, only Day Gain/Loss figures are not. At the same time, the mobile version has it right. What!?
https://community.quicken.com/discussion/comment/18662420#Comment_18662420(edited to include a previous thread about the topic)