Why do LED bulbs lose brightness when turned on?
In any case, you need to check the quality of contact connections. Perhaps the problem is poor contact. And incandescent lamps, this problem is not terrible - and therefore burn normally.
It is also possible to increase or decrease the voltage in the network or some kind of interference, the incandescent lamp, in turn, takes on these interference, providing a kind of stabilization, if we formulate it in simple language and not get into the wilds.
It is also worth considering the quality of the lamps themselves, have they not used LEDs before? Maybe you should try to buy a light bulb from another manufacturer and see how it will behave?