Because before version 3.50 tint used to be stored in BGR instead of RGB. https://github.com/photonstorm/phaser/blob/v3.24.1/src/gameobjects/components/Tint.js#L14
Given that it was applied automatically in the setter, but not reversed in the getter, the value reads as different one than what was set.
Either convert to RGB yourself, or update to the 3.50 beta version where it’s no longer an issue.