CSS in robot tests contains 0x01 values

I'm running into an issue where the CSS in robot tests - at least for logged-in- contains some weird stuff.
Does anyone have a pointer to where this could come from?

See https://github.com/plone/Products.CMFPlone/issues/2579

No idea but I would debug to check what's going into/out of the plone.resource folder when CSS is compiled and what's served via WSGI.