Users may configure their system in such a way that Qt computes a scale value less than 1. If the program enables pass through scaling policy to enable fractional scaling this breaks previously correct behavior for these users (because now their 0.X is not rounded to 1).
It seems there is already some discussion of this issue in the bugs and but no fixes have been submitted . But I consider this breaking since it means developers must now choose to support fractional scaling and break some users or disable fractional scaling for all users. If sub 1.0 scaling values are unsupported, then Qt should round these up regardless of the scaling policy so that developers can enable fractional scaling for cases Qt actually does support.