Can't create the example mapping rule

I tried to add the two example mapping rules in the mapping rule creation page, but failed to create the simpler example. I suspect this happens with similar rules. The failure to create or save the rule is not reported in the UI (nor a message saying it has been saved appears).

Please check out the attached video for reproduction.

This has been tested with current SaaS, but I think it will be present in other on-prem releases.