I think I have managed to replicate this. 
I couldn’t on my existing test site, but wondered if it was because I had already done/changed/tweaked something previously that may have affected it, so I spun up a new one and tried it on that.
- Created fresh site
- Enabled chat, and added
everyonetochat_allowed_groups(alongsidestaff) - Signed up and logged in three test users, auto-enrolled them in ~general channel (and made one admin). Logged them all out.
- Made a cup of tea
- As main admin, ensured they were members of the channel and pinged them with
@all
- All received notifications
- Only the admin user was sent an email about it
