Minus sign encoded incorrectly for some locales
Minus sign ('-') for effect parameters (and possibly in many other places, too) does not work with some locales where it is encoded differently by QLocale (used by Kdenlive refactoring version) and locale library in C (used by MLT).
This sign is saved to project file with locale-dependent encoding. In hex, it is encoded by QLocale as '0x2d' for en_US.UTF-8 locale, but '0xe28892' (3 bytes) for some others, like et_EE.UTF-8 (Estonian) locale. The bug might be reproducible with all the other locales that encode minus as anything else than '0x2d' while using QLocale.
As Shotcut and stable version of Kdenlive do not seem to be using QLocale, they encode minus sign always as '0x2d' like MLT and therefore the negative values work correctly there.
The result is that after setting these parameters to negative values in Kdenlive, their effective value stays at 0. This can be reproduced even without saving the project, as the problem exists already in the internal model, not only in XML file. Raising the values above 0 works as expected.
An example effect that can be set to a negative value is "Volume (Keyframable)".