After some tracing and hard thinking Taco and I found interesting spaces showing up in the output that result from a kludge in tikz:

Normally one will use \ignorespaces to catch spurious spaces but tikz invokes \nullfont assuming that its fontdimen 2 is zero. But, in context it no longer is (prelude to delayed font loading).

You can put this in cont-new.mkiv as a temporary hack

\let\normalnullfont\nullfont

\def\nullfont
 {\fontdimen1\normalnullfont\zeropoint
  \fontdimen2\normalnullfont\zeropoint
  \fontdimen3\normalnullfont\zeropoint
  \fontdimen4\normalnullfont\zeropoint
  \fontdimen5\normalnullfont\zeropoint
  \fontdimen6\normalnullfont\zeropoint
  \fontdimen7\normalnullfont\zeropoint
  \normalnullfont}