I have Saturn in the 12th, and have followed my young audlt daughter's life with interest: she has her sun and Mercury in the 12th.
I agree that 12th house people need their privacy. Traditionally the 12th rules shut-ins, notably people confined to hospitals, prisons, or monestaries. Well, obviously most of the millions of people with strong 12th houses are not sick,in jail, or under holy orders, but I think they need their solitude.
In terms of "self-undoing", one can trace the meaning of the sun through the 12 houses to get some comparison. In the 2nd house, one identifies with money, possessions, or values; in the 7th house, with one's marriage/partnership; in the 10th, with one's career/public image, and so on. But the 12th house ends this cycle, and was traditionally associated with the end of life (though the 8th plays a role in traditional astrology here, also.) It consequently was regarded as misfortunate, something that modern astrologers thankfully ignore. But the 12th is where the energies of whatever planets are there get "undone"--if the sun, in a Neptunian kind of way, it is one's ego that gets dissolved.
Now 12th house people are not necessarily shy loners: many famous political leaders have 12th house suns! But they do best by identifying with a cause greater than themselves, which might be their constituents, political principles, or their party.
Leo planets in the 12th at first glance would seem out of place here, but Leo as a leader does well by identifying with some greater cause that attracts other people to it. For example, Hilary Clinton has a strong 12th house. Love her or hate her, but her presidential campaign speeches indicate that she thought she was running in support of ordinary American citizens.
Of course some people struggle with the 12th house, in that they would prefer to identify with their career, friends, or hobbies. But life seems to go better for them when they identify with an altruistic purpose. Especially with any kind of 6th house emphasis, service to others will often bring the recognition denied by focussing on the affairs of other houses. This is why some of the great humanitarians like Mother Teresa have had strong 12th houses.
Dutch astrologer Karen Hamaker-Zondag wrote that 12th house planets indicate that one's mother had a difficult labour and post-partum experience. This was not true in my daughter's case, however.