I’ve discovered that some modifiers in 2.8 force a mesh’s texture space scaling and position back to auto-generated rather than manual control, such as bevel and the new options for mirror. Is this a known bug or is it intended like this?
As modifiers changing mesh bounding box, auto-generating another automatic texture space is probably intended behavior.
But fact that auto-generation is not deactivated when texture space is custom and not automatic is probably an oblivion.
You should fill a bugreport about that.