PDP-8 GTF question

I need to pick the brains of some PDP-8 experts. According to the references I can find, especially the Small Computer Handbook, the GTF instruction should include the M837 interrupt inhibit bit in AC3. However, maindec 8E-D1HA test 05 seems to depend on this not being true. Running the GTF instruction on my 8/M when the inhibit FF is set results in a 0 in AC3. Furthermore, both simh and Doug Jones' emulator omit the interrupt inhibit bit in the GTF instruction. In fact Dr Jones' emulator has a /*?*/ comment in the code. Does anyone know the origin of this discrepancy? Was the behavior the same on pre-Omnibus versions of the 8, or did earlier address extensions include that bit in the GTF result? I came across this oddity in the course of writing an emulator of my own, and I'd like understand what's going on here a little deeper than "ignore the documentation, and make it work like the real hardware." Thanks in advance, BLS