On a Slack workspace where there are custom emojis with non-ASCII characters (in my case: :giàfatto:, :èinaccettabile:), they are displayed correctly:

• in the emoji/reaction picker (filter and selection work fine)
• in the emoji completion dropdown (prefix filtering works fine)

but not

• in the message body (they remain unexpanded)
• in the reaction "button/count" itself (the button is displayed with the reaction count, but instead of the image the first characters of the emoji name are displayed)

Thanks, I will fix this.