"I know the IRS will definitely flag this". No. It will not get flagged if it is included on line 3 (rents received). Then take a deduction on line 19.
"But can I really not e-file because of this"? No. You can't efile if you attach a separate statement. Line 19 includes an e-fileable statement, if there are too many words to fit on line 19 (I would keep it simple "Jan 2020 rent not rec'd in 2019')
