I have had this happen once on a comment post. (Also tried multiple edits to resolve, but it didn't work.)
Connect for Lemmy App
A community for the mobile app Connect for Lemmy.
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First post on a new thread being this comment or the first comment on a post, like yours? If yours counts and mine doesn't you should've put a touch of markdown in your comment as more of a test.
Also, markdown is different in different places. The markdown Reddit uses, vs Lemmy, vs Discord are 3 different lots. Maybe the broken bits you see don't work on lemmy?
image looks like underscores either side so maybe that's the issue?
Edit: underscores are italics? Why are there two italics options lol
Damn. "First comment" theory is out. It's something though.
I have had this issue show up on my own comments as well. (Even with using the I function in Connect or not.)
Also underscores have been italics for years across many platforms. It's not always, but is generally. (Well, I have been using underscores as a pseudo-emphasis before markdown was even a thing.)
Edit: From a now deleted post, but you can see it in my history: