And even worse, 5.6.0 to 5.7.0 is, by semver, one minor point release to another minor point release - no breaking changes, no major bugs. 5.7.0-pre would raise some flags.
> A pre-release version MAY be denoted by appending a hyphen and a series of dot separated identifiers immediately following the patch version.
Note that it says "MAY" and not "MUST", so it's optional.