The bug
It seems as though underlines in chat intersect neighbouring characters. When the text fades, this shows an inconsistent dotted pattern where they intersect.
How to reproduce
Take a screenshot
Watch carefully when the text disappears
Example
[media]Linked issues
duplicates 1
Attachments
Comments 15


This is not a problem with underscores, it's a problem with underlined text.
Can confirm for 1.13-pre4, however.

Still in 1.14 Pre-Release 1

Still in 1.14 pre-3, 1.14 pre-4 and 1.14 pre-5

Still in 1.14 Release
Can confirm in 1.18.2.
Can confirm in 1.19 and 22w24a.
Can confirm in 1.19.2.

Thank you for your report!
We're tracking this issue in MC-108323, so this ticket is being resolved and linked as a duplicate.
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We're resolving this ticket as a duplicate of MC-108323 after some internal discussion since it's the same issue. 🙂
I'm assuming you're talking about underlined text, not the underscore.