My Mom never leaves her room, yet has spent the last 1.5 hours hanging out in the living room with my son....I feel "punked"! Thoughts?

Sounds utterly normal to me. She knows you, is used to you, can see you any time she wishes. A visit from the grandson is special. I honestly cannot see the problem you have with her behavior at all. She is following her personal habitual way of living; I think it frees you to have your own privacy.
However, if you feel slighted why not discuss it. Why not tell her "Mom we would love to have you out here with us, sharing programs and meals and time talking together. If you would love it, as well, just know you are welcome any old time. If not, and you would rather have your own time alone, we understand. We want you to know you are free to do whatever makes you happiest. But we are always here, and would love your company."
Not much else to be said, I think.
And yes, I have a grandson I seldom get to see, and I surely can understand putting off my meaningless habits for him when he visits.

One thing I know about eldercare is that I don't know ANYTHING.

My mom acts one way with one person, a totally different way with someone else. I have a couple of ladies in my neighborhood who know mom from the Sr Center. It was 12 YEARS before they put together that she was my mother. She never, literally never talked about me to them, and for at least the first 7 years 'knew' that their neighborhood was also mine and that I saw these ladies 2-3 times a week.

Still SMH about this, and many other oddities.

AsianDaughter
It goes with the territory. My DH aunt had an event in her home that caused her to need to evacuate. I was dreading telling her that she had to go by ambulance to an ALF while the home was being repaired. She’s 95, on hospice and Bedfast. She acts like she got a get out of jail free card. She can’t say enough good things about the place. She is a poster child for them. Who knew? Now I am trying to figure out how to stretch her funds so she can stay there, afraid a more affordable place wouldn’t be as appreciated.
In your moms case I would be happy that she feels comfortable to do as she pleases and enjoy your free time. One thing about the declining and dementia, it progresses. Whatever it is today will change.
Count your blessings and see if the son can come around more often.

ZippyZee’s reply reminded me of a patient I sat with one day when I was a hospice volunteer. When I arrived at his house his wife and daughter both said “Joe” will stay in his room and you will just need to look in on him once in a while. Fifteen minutes after they left, “Joe” came out to where I was sitting and talked with me until they came home, a total of four hours. His wife and daughter were happy to see he was able to have a good visit with someone else.

I saw similar situations with other patients too.

So, yes, a change of routine does sometimes make a difference. Be happy for it, AD.

It's a special occasion and she's making a special effort. Try to look on it as an honour marking your son's visit, rather than any reflection on how she feels about her everyday quality of life.

You know what, though. If your mother was in your space every day of the year, never absent from the dinner table, expecting you to take her on outings, sitting in your living room through the long evenings... you'd soon be wishing she liked her own company better.

How long has she been living with you?

Can't really say what goes on in the mind of a Dem/Alhz's patient. Maybe a sudden change in routine and having a new person around inspired her to get up for a day? Grandma's love their grandsons pretty strongly. Just be happy she had a good day, and try not to take it too personally.