Caused by rBc476c36e4008.
This hooks into the existing FIXME (workaround for a missing update
tagging from operators), needs to also check the shading.flag (to detect
changes in V3D_SHADING_MATCAP_FLIP_X).
While this fix works, we have a couple of related reports, namely T74872,
T74744 where the NS_VIEW3D_SHADING notifier is not working reliably. Not
sure if there is a more general fix?