Bugs in LibreOffice 7.6
Start with reading the How to use this Ask site? guide; in its Bugs section, it discusses how to report bugs.
Then re-read the description of the downloads offered on the Download page. Specifically, notice that 7.6 branch is (as of today) only recommended to early adopters; 7.5.7 is the version that is tested longer. So if you use LibreOffice in your business, it would be wise to use more tested version.
Then please read the great Simon Tatham’s essay on How to Report Bugs Effectively. E.g., one would love to know what was the exact contents of
B4:D4; what is the sequence of clicks, and where you drag to; what exactly you see as the result; and what was your expectation. Or better, a sample file would be great, accompanied by e.g. a screencast. And of course, show that you did your own research, e.g. that you read the respective help sections - on extending selection (autofill) (and note the “If you select two or more adjacent cells that contain different numbers, and drag” part), on relative addressing (relevant if your
B4:D4 contain formulas).
See another help topic, devoted to the SUM spreadsheet function. Specifically, note the “This function ignores any text or empty cell within a data range” part.