The issue resolves around inserter balance where two inserters are trying to grab a resource from an assembler. In the old version, as the output resource was produced by the assembler each inserter would take turns to get an item from this assembler and thus it was balanced. But in version 12.30 I noticed now that one inserter can dominate the resource i.e. grab the output from assembler without letting any other inserter take the resources.
Here is an example of a setup where this occurs:
