Got another new 70W bulb recently, this one flashes in the arc tube and not anywhere else but does not sustain an arc. Once again, all connections and sockets (it's a screw base lamp) good. Same gear, no issues with a Philips 70w reflector CMH though.

Is it likely the lamp is defective?

It looks like too low ballast OCV (here I mean voltage without the ignitor pulses) for the given tube, or tge ignitor may have degraded amd is firing at wrong phase (normally it should fire just before the peak in the 50/60Hz sinewave voltage, so the discharge will more likely sustain with high enough currents to warm up electrodes).
CMH's are generally lower pressure and shorter electrode gap devices, so it is possible they get away and suffice with lower OCV or more off phase ignitor.

Well, just found out one of the internal wire connections near the base has broken off leaving about 1 mm gap, which probably explains why the ignitor was able to make the arc tube glow slightly but the main arc didn't strike...

@1mm gap: That explains everything, indeed... :-D