I've read that the Ukrainian war is a problem for China for exactly the opposite reason: the invasion of a sovereign country by another is somehow like outside

I've read that the Ukrainian war is a problem for China for exactly the opposite reason: the invasion of a sovereign country by another is somehow like outside countries (the US, Japan) supporting Taiwan, which China claims is a part of their country rather than an independent country. I'm not sure I really buy into that argument, though; your suggested viewpoint seems at least as valid.