Inquiry aging should go off of the date of the inquiry rather than the 1st of the month like age of accounts factors, but to my knowledge an inquiry reaching 3 or 6 months is a non-factor with scoring; I've always believed that at 365 days 100% of the points lost from the inquiry a year prior return all at once.
OK, so I doubled back and checked the inquiries. In the December 18th report there were 3 inquiries due to drop off (from the less-than-1-year group) at different points in December, but those same 3 inquiries still showed up in the December 20th report, so I don't think inquiries falling off was a factor in the score change.
Total revolving limits 741200 (620700 reporting) FICO 8: EQ 703 TU 704 EX 687