Let me try to clarify: on my system, one inside vertex on the fixed 2019 template is incorrectly detected as outside by inpolyhd before openmeeg is called. (You said that didn't happen on your Windows 10, so that's what's inconsistent.)
However, when using "force inside skull", the vertices are "re-centered" before calling inpolyhd. In that case, I get the correct answer that all vertices are inside.
Have you simply tried to replace inpolyhd with inpolyhedron in tess_force_envelope.m?
Yes, and it gives the correct answer whether re-centering or not.
How does it compare in terms of execution speed?
inpolyhedron seems 35-40% faster on my system.
Does it move a lot of vertices when ran on the ICBM152 template, or just the one you wanted to move?
None: it correctly detects them all as inside.