Ok, figured it out.
The segwit parts of my blocks were wrong, so most nodes would reject them.

However, there seem to be some old nodes that accept erroneous blocks like that (being unaware of these parts thanks to backwards compatibility), including that block explorer, but they don't propagate into the more up-to-date part of the network.
That's where the impression of segmentation came from.

I eventually stumbled onto a node that gave me a proper reject message (bad-witness-nonce-size), and it all clarified from there.