I input the following as the message to be said:
\*italics\* \*\*bold\*\* \*\*\*both\*\*\* \_underline_ \~strikethrough~
And the config file registers it as the following:
\\*italics\\* \\*\\*bold\\*\\* \\*\\*\\*both\\*\\*\\* \\_underline_ \\~strikethrough~
That's all fine and dandy, I suppose, escaping the backslashes, but when I close out of mine menu and reopen it, the "Configure Entry" screen looks like this:
\*italics\* \*\*bold\*\* \*\*\*b
I suspect this is most likely due to the slashes.
So, there should be some processing there to convert the \ to \u005C, as well as other characters. I can confirm this works because an apostrophe I put somewhere got converted to \u0027.

@wafflecoffee
I actually didn't do the config system - Report that at shedaniel's cloth config I guess?
Edit: Could maybe try to fix that though

Oh wait a minute, I think that text just doesn't fit the textbox - you can "scroll" in that thing with the arrow keys

No; when I execute the order it only says the part up to the \*\*\*b

You can download a fix here: https://github.com/UltraBlackLinux/MineMenuFabric/actions/runs/827104336