Peter,

Groups.io may be the only organisation that has chosen to embed a "+"
within leftsides of addresses within it's own domain, ...

Gmail is another.

It uses + in the user part as the lead-in for an alias address. That feature won't work properly with messages sent from a synchronoss afflicted service.

so we may get no support for any campaign to get it fixed properly.

You can add to your leverage by telling them that they're breaking a Gmail feature.
https://support.google.com/mail/answer/22370?hl=en
(the "Use Gmail aliases" section at the bottom of the page)

Shal