How is it a problem if the MC is located in the twelfth house?
I tend to think of it as just another layer of meaning. If the MC is not located in the tenth house whole sign--or the tenth house equal house, if the ascendant-based equal house system is being used--that just puts a hint of some other house into the native's career and public reputation. For example, Eleanor Roosevelt's MC was in her eleventh house whole sign--in Libra--and she's a shining example of someone who made her public reputation in an eleventh house and Libran way: focused on social justice issues.
Any way you could equalize the houses for people born at high latitudes is usually not going to put their MC in the tenth.
I recently saw a chart on here with the MC and AC in the same sign. The native was born in Iceland, in the month of January. Classic example of wildly unequal houses.
It doesn't need to be a problem, unless one is into traditional meanings of houses. I practice modern choice-centered astrology, but I try to be informed by traditional astrology. Of late, it guides my thinking more than the more extreme forms of esoteric and New Age astrology do. So if we're not doing the "anything goes" approach to modern astrology, I find it helpful to consider the meanings of the different houses.
Traditionally the 12th house was a Big Pile of Hard Luck. More modernly, possibly due to the influence of Hindu astrology, it is a house of mysticism, altruism, seclusion, and the blind spots we each have about ourselves. It is hidden enemies, hospitals, and self-undoing. But also where, if we choose empowering interpretations of the 12th, we can do a lot of good for people less fortunate than ourselves.
The meaning of the MC, in my book, is just a lot more consistent with meanings of the 10th house (vocation, public image) than it is with meanings of the 12th. However, if the ruler of the MC/10th house is in the 12th house, then its meanings will be pulled in, for sure.
I have a different sense of Eleanor Roosevelt's contribution. I wouldn't see either Libra or the 11th as classically associated with social justice issues. To me, the 11th house does not somehow equal the sign of Aquarius. The 11th is the house of friends and associates, as well as one's hopes and wishes for the future. Which fit her, in a slightly different way.
But here is Eleanor Roosevelt's chart:
https://www.astro.com/astro-databank/Roosevelt,_Eleanor
Her MC is in Libra, with Venus in the 9th house in Placidus. In whole signs, her MC is in Libra, in the 11th house, with Venus in the 10th.
With most of Roosevelt's planets above the horizon, the sun conjunct her NN, and that big caring Cancer moon, she looks like someone who needed a public life, with concern to care for others, and some pretty advanced thinking (Mercury conjunct Uranus.)
Some high latitude births do have more "normal" houses, but I've not taken the trouble to determine what times of year this happens.
I spent a little time searching high latitude births once. These two charts, both for a Finnish prime minister, illustrate the problem. I note that he was born in February, after the solstice-equinox midpoint. He had his difficulties, but not consistent with core meanings of the 12th house.
https://en.wikipedia.org/wiki/Ahti_Karjalainen