I put a string of about 8 emoji characters (a few outlines, a few colored emojis) after the leading sentence that did get posted. The software ate everything else in the post starting with the emojis.

On my site, I get the following error page come back when I attempt to post a single smiley face in a message body:

Database error!

Incorrect string value: '\xF0\x9F\x98\x80' for column 'text' at row 1

The collation has been set to utf8mb4_general_ci on all tables and columns.

What should I try?

It kind of looks like a problem that has not been worked out in MLF, however, since your site chokes on emojis in a different way than mine.