These sorts of changes do in fact happen fairly frequently and developers need to be aware that they can't blindly accept upstream dependency changes.
Because Rails explicitly has a versioning policy where minor versions are equivalent to SemVer major versions (but with deprecation notice in a previous minor version) and where major versions are also SemVer major with subjective significance distinctions; they call it “shifted SemVer”.
https://guides.rubyonrails.org/maintenance_policy.html
This is somewhat obnoxious, but not as bad as saying you use real SemVer and then brazenly breaking things in minor releases.
Changing semantics of a query operator is something worth saving for a "big major" update, if you change it all.
Some breaking changes are really obvious and easy to catch. Others can introduce pernicious bugs which slip through tests. The change Mongoid is solidly the latter. And yeah, they say they use actual Semver.
Arguably, it was so important that they did a big major update just to deprecate it.