answer is similar to https://forum.mendix.com/link/questions/90904
Solution was to open a quote on one line and close it on the next, resulting in the line break being generated:
$Proposal/AuthorFullName + '
' + $Proposal/AuthorStreetAddress + '
' + $Proposal/AuthorPostalCode + ' ' + $Proposal/AuthorCity + '
' + $Proposal/AuthorCountry
I would like a special function called AddLineBreak that would insert this for better readability of code.