By your logic PRC "reunifying" with TW wouldn't be met with sanctions because it's a civil war where TW is de jure
recognized as part of Chinese territory, at least by parties who can do meaningful sanctions. After all PRC can't annex what's already legally hers.
Except sanctions will be driven by political decisions in democracies. I feel like most of the EU and US citizens would consider it an invasion - so sanctions.
It's not an international law driven by unflinching rules. It's a democratic choice, or a series of them.
I think people are viscerally responding to the invasion and bombings, less so the political status of a region. Annexation isn't the distinctive feature here. Lots of people protested the Iraq war, around the world, and it didn't have anything to do with annexation. Civilian lives is what matters.
By your logic PRC "reunifying" with TW wouldn't be met with sanctions because it's a civil war where TW is de jure recognized as part of Chinese territory, at least by parties who can do meaningful sanctions. After all PRC can't annex what's already legally hers.