That's always been a 'feature' of the two PIR controlled floodlights I've wired via an unswitched FCU. Presumably, by design, re-energising the mains feed triggers the one shot timer used to keep the lamp lit after being triggered by the motion sensor specifically to allow you to verify that the replacement lamp is functioning during daylight (the most likely time for such a lamp replacement exercise from atop of a ladder).
I think I may even have seen instructions about using an inline switch to signal a permanently on mode (possibly mediated or not by the dusk to dawn light sensor) by a rapid on/off switching sequence (possibly 2 or 3 cycles) but I've never made use of this feature, just the one where switch on after relamping lights it up regardless of the motion and dawn to dusk sensors.