....I would have expected so but because your comment had a question mark, I decided to check it out of curiosity and very unexpectedly when I mount together my M5 + Adapter + Kenko 1.4x + 40mm 'pancake' F/2.8, I can still set an aperture of F/2.8 according to the M5 screen and viewfinder. So, testing my other combos of relatively small lenses (not telephotos) :
M5 + Adapter + Kenko 1.4x + EF 100mm F/2.8L opens to F/4 widest < As expected when Extenders/Teleconverters are mounted.
M5 + Adapter + Kenko 1.4x + EF 24-70mm F/4L opens to F/4 widest < According to screen and the same behaviour as the 40mm pancake combo.
I don't fully understand this and am puzzled. I wondered at first if the Kenko Extender doesn't report properly to the M5 Canon body but then such behaviour is not consistent between the EF 100mm and 24-40mm both F/4L lenses.
It's not a problem to me in any event because with Macro work I'm never shooting as wide as F/2.8 and anyway reading the light at F/4 when the Extender is mounted has never adversely affected the resulting image.