Yes undersized neutral was common when the high power loads were all resistive or inductive, like heating and motor loads, so the neutral only had to carry a difference current equal to the household current rating over the 3 phase set of connections. Typically was 60% of the conductor diameter for the phases, and the same for the PE conductor, though TNC-S uses the same conductor instead. done when copper prices peaked, to lower the cost of building out networks for the utility operators and municipalities. That changed when switching supplies became common, and the first use was in TV sets, where you suddenly had large DC current flows in the neutral, and this accumulated as the TV sets tended to turn on the thyristor power supplies late in the voltage waveform, so the neutral current was massive. Led to quite a few substations blowing up as the DC current flow through the windings saturated the core, and then in turn the primary side current was not limited, except by the winding resistance and the lower air core inductance, leading to massive heating and failure.
The current thing is to size neutral to the same as phase conductor, though with increasing use of PFC for larger loads the neutral current ca be lower, but there are still large numbers of loads with poor power factor, including LED lights, especially those with trailing edge dimming, as you get pretty high current flow where the corresponding other 2 phases are not going to draw current out via the neutral. A lot of cheaper appliances also forego PFC, because that makes them cheaper, and thus you get the high peak current pulses.
If the high mains is all of a sudden, and there are no new developments by you, then it is a failing neutral, and you are on the lowest loaded phase, or the neutral at the substation is burnt out already, and open circuit, or the cable has been stolen. Complain to the power company again, and to your local municipality that your house and equipment is at risk, and that you will have damaged equipment from this, along with the increased risk of a fire from failed equipment.