It is as Ziechael said, by default line breaks is interpreted by Roll20 as a new command, and is mentioned at the start of the Text Chat article on both the Help Center and the Community wiki: Multiline Chat Commands
You can hit shift enter while typing in chat commands to get to the
next line. Each line will be individually interpreted allowing you to
send multiple styles of chat together, preventing them from being split
in the chat by other player's messages.
/me strikes a valiant pose.
Evil doer, beware, I am on your trail!
/w gm This is the signal that the thugs I paid are waiting for.
It's possible you've missed this aspect of how the Text Chat work before when writing other messages with line breaks, and it only became apparent now when you used a whisper command. I'll add a warning to the community wiki's page in the whisper section for people to remind of the Multi-Line/line break thing works. Ziechael said: [...] encase the entire thing after the /w command in {{ }} (you'll get the {{}} as part of the output but it will preserve the multiline output). /w whoever {{blah blah blah line/paragraph 1 blah blah blah line/paragraph 2 blah blah blah line/paragraph 3}} Wow, this is new to me! Also adding this "{{}}"-trick to both the "Multiline Chat Commands" and "Whisper"-section, it 100% should be mentioned as an option. I might have encountered this in association with rolltemplates at some point, but must have shrugged it off as a feature related to them, instead of being a general thing.