bycol should give 1 row as an answer, but is giving me the same number of rows as its input?!

I have been building up a spreadsheet and one of the things I've been working on is a summary of the prices for the deliveries I've been doing so I can accurately charge my customers.

Oddly, at the end of the equation in one of the cells is;

BYCOL(tmp, LAMBDA(c, TEXTJOIN(CHAR(10), TRUE, c)))

Which by all rights should only give 1 row of output, yet it's not doing that. I'm getting the same number of rows in the output as tmp has as the input, which is utterly confusing me.

The other issue I'm having is that if I use;

HSTACK(MAKEARRAY(ROWS(tmp),1,LAMBDA(x,y, TEXT($N$1, "DD/MM/YYYY"))), tmp)

It puts the list of dates on the left as it should but the last column of tmp is now #N/A instead of what it was before the HSTACK.

Tearing my hair out here trying to work out what, if anything, I'm doing wrong....