I'm using the markdown editor GUI on PieFed.
A comment I wrote looked OK on PieFed, despite me having doubts because I didn't add spaces at the end of lines to force line breaks.
Today I saw it in Mlem, and line breaks were as expected, not as I've seen them on the PieFed web UI. On Lemmy, they are the same as in Mlem.
PieFed: https://piefed.zip/comment/2945722
Lemmy: https://discuss.tchncs.de/post/51439392/22823110
Mlem shows it like Lemmy. No screenshot necessary.
Summary: In Markdown, you need to add 2 or more spaces at the end of a line to force a line break. I didn't do that. A client app and a Lemmy instance show the text correct without a line break. PieFed adds a line break.
Update: I'm terribly sorry for using the word “standard” in a non-standard way. I mean this: https://daringfireball.net/projects/markdown/syntax#p
Update 2: Created issue #1524.
Putting standards aside, I personally prefer aligning with Lemmy.
On my own server, I handle it in the following way, though I understand this is largely a matter of personal preference.
https://codeberg.org/karasugawasu/pyfedi/src/branch/main/app/utils.py#L666