The original was actually this github issue for Linux Mint Cinnamon, last seen 2019-06-22.

All I really cared about was the syntax for sending notify-send multiline notifications… like this:

notify-send -i error "my summary" "a bunch of text.\rand some more text"

It is possible to display multiple lines with notify-send.
BUT
There is a bug in the cinnamon version.
All examples work CORRECTLY in xfce.
In cinnamon, failure is both summary and body line length dependent.
48 characters or less in the body
AND/OR
6 characters or less in the summary
Examples:

notify-send -i error "summary" \
"summary=7 body=48 --FAIL--\
\r123456789012345678901234567890123456789012345678\
\r123456789012345678901234567890123456789012345678"

notify-send -i info "summary" \
"summary=7 body=49 --OK--\
\r1234567890123456789012345678901234567890123456789\
\r1234567890123456789012345678901234567890123456789"

notify-send -i error "summar" \
"summary=6 body=49 --FAIL--\
\r1234567890123456789012345678901234567890123456789\
\r1234567890123456789012345678901234567890123456789"

notify-send -i info "summar" \
"summary=6 body=50 --OK--\
\r12345678901234567890123456789012345678901234567890\
\r1234567890123456789012345678901234567890123456789"