Why does drag cause merged cells to demerge ?

Hi. I’ve noticed an odd behaviour in Calc. I have a couple of spreadsheets where I used merged cells to hold longer data strings. Cells in row 1 are not merged and show various totals. On each row of the sheet from row 2 down, column A holds a date, columns B,C & D are merged to hold a text description and columns E,F & G hold numerical values. If I have a text description in row 2 (ie in merged cells B2-D2) and try to drag it down into lines below, all the lines below get the data but the cells demerge and the data appears only in cells of column B, with individual cells in columns C & D reappearing. Is this normal behaviour and something I have to live with, or is it a bug of some sort ? I get the same result on all my versions of Calc.
LO 6.0.7.3 on Linux Mint 19.3 (32bit) on a 15 year old laptop rescued from a bin - a bit slow but works fine,
LO 6.4.4.2 on Win 10 2004 (64bit) PC intel core i5,
LO 6.0.7.3 on Linux Mint 19.3 (64bit) another 10 year old PC someone was going to throw out.

After drag, use the Clone Formating tool (the paintbrush).

There is a similar question that I can’t find right now.