multiline currently defines max-lines like this:
max-lines - This defines the maximum allowed number of lines in a multiline batch RECOMMENDED
I suspect that this means the number of PRIVMSG/NOTICE commands in the batch, not the number of message text lines after processing multiline-concat, but as written, it could be either. The spec calls both of these 'lines' in other places.
I don't have a concrete proposal for improved language though, so filing this as an issue rather than a PR.