An iPhone 8 Plus can't send / receive MMS messages (i.e. non-iMessage group messages) when on WiFi calling on Verizon - it just sits with the progress bar when trying to send and then eventually fails.
Phone calls and individual SMS messages work fine when on WiFi calling, and going down the street to where there's 4G signal causes MMS to work immediately.
A Galaxy S9+ on the same WiFi calling is able to send/receive fine. The underlying internet is 1Gbps fiber and works well.
In case it's relevant, there is extremely marginal cell tower coverage at this location (i.e. ~1 bar).
Updating iOS on the phone did not solve the issue.