Google is rejecting it to ensure incoming messages aren't spam. SHOULD means "you should do this unless you have a really, really good reason not to." Do they have a good reason not to? It doesn't seem so, meaning Viva is in the wrong here.
After all, linguistics is full with examples of words that are spelled the same, but have different meaning in different cultures. I'm glad the RFC spelled it out it for everyone.
On the other end, we may receive messages with or without. Both are valid. We MUST therefore accept both variations.
The second one is a consequence of the former. So yes Google is the violating party.
When the docs disagree with the reality of threat-actor behavior, reality has to win because reality can't be fooled.