Yes, it is uncommon, but it is also a "gotcha" - as the manufacturers do not (usually) advertise their full frequency impedance curve...
All you can say, is that if running 4 ohm (nominal) speakers... you should be cautious, and typically, 8ohm (nominal) speakers, are pretty safe.
So I do tend to warn people ... the low impedance thing can drive you nuts, as the system doesn't sound right, but there is no obvious reason for it... which can lead to random upgrades, and discussions about how different amps "sound different" (yeah they often misbehave differently under low impedance conditions, and their distortion patterns differ...).
And then you try an amp that is stable into 1ohm loads, and suddenly the midrange clicks into place, imaging stabilises, dialogue is intelligible.
It is a frustrating and pernicious problem... (if you have it) - and typically you can only uncover that root cause by finding a proper lab test of the speaker where the full impedance/frequency is mapped....
(and just to exacerbate the problem, most amp manufacturers don't publish data on their 2ohm or 1ohm capabilities....)