Can you reproduce this issue in a new project? Can you reproduce this issue in an older Unity version? Do you have more than one Unity editor open?
If something resets when entering play mode, it's often related to Unity's assembly reload, where it re-initializes a lot of things. It often reads settings via EditorPrefs, to restore settings, which is shared among editor instances.
You probably tried this already, but just to make sure.
Close all Unity editor instances
Open a single editor
Set slider to 1x
Close editor
Open your project and check if the issue persists
This should make sure that Unity writes this setting to disk/registry. So if an assembly reload occurs, Unity should pick up the extising values stored on disk/registry.