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That's the problem.

most specifically the problem is that markdown tables don't allow breaking the table row in multiple lines

but then you can always use HTML tables in markdown and Pandoc transforms it just fine

Every markdown implementation is supposed to allow inline HTML.

<br> has worked fine whenever I’ve needed line breaking in markdown tables

They mean in the Markdown code, not in the output.

I think they mean breaking the line in the markup, not the output

yes, that's what I meant, indeed.