External o365 Mail-Users Receive plaintext

In my Office365 I have an Office365 Group. Whenever I add a Mail-User (external) to the group they receive the welcome message from the group, but it is in plaintext, complete with image file attachments and winmail.dat.

The message comes across like this:

Welcome to the Roadkill Cafe group.

[cid:office365_icon_orange]

[cid:group_photo.jpg] Roadkill Cafe Welcome to the Roadkill Cafe
group.

Roadkill Cafe group

[cid:welcome_onenote_icon]https://outlook.office365.com/owa/roadkillcafe@roadkillcafe.com/groupsubscription.ashx?realm=roadkillcafe.com&source=WelcomeEmail&action=onenote
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noteshttps://outlook.office365.com/owa/roadkillcafe@roadkillcafe.com/groupsubscription.ashx?realm=roadkillcafe.com&source=WelcomeEmail&action=onenote

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[cid:document_icon]https://outlook.office365.com/owa/roadkillcafe@roadkillcafe.com/groupsubscription.ashx?realm=roadkillcafe.com&source=WelcomeEmail&action=files
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fileshttps://outlook.office365.com/owa/roadkillcafe@roadkillcafe.com/groupsubscription.ashx?realm=roadkillcafe.com&source=WelcomeEmail&action=files

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I have tested this with my local email server and with Yahoo.

However, if I add one of the non-external users (licensed o365 user), the message is sent in html. Or, if in OWA, if I add a "guest" to the group who has a yahoo.com address, that person is also sent a normal html email like it should. I have also tested this with a gmail account.

So I added a remote domain under Exchange > mail flow > remote domains and configured that domain for Always use rich-text format (it was default set to follow user settings) and I changed the character sets to Western European.
But this had no effect.

This MS help document says to make an adjustment that will force the message into plaintext and eliminate the winmamil.dat file altogether. However, I want the message to be in html, not plaintext. And besides that, none the three settings (Use rich-text format: Always, Never, Follow user settings) have any effect on the message at all.

How can I fix this?

[update]

Apparently, the mail-user receives ALL email correspondence in plaintext, not just the welcome message. This happens when you post anything in "conversation" in the group itself.

However, if I send an email to that same group, it is received as html! Go figure that. Post to the group directly and it sends plaintext; send an email to it and it sends html.

[update]

So I tried using powershell against the mail-user object like this, but this also did not change the welcome message to html:

set-mailuser -identity bob.newhart@roadkillcafe.com -messageformat mime -messagebodyformat html