In case anyone else experiences this...

I have an itinerary with a domestic connection to an international flight (and the converse on the way back). The domestic segments are in Economy (Plus) while the international segments are in Premium Economy.

When I tried to apply PlusPoint upgrades on the website or in the app, I could only apply the upgrades to the international segments. The website and app showed no upgrades available for the domestic segments when trying to use PlusPoints.

I called the 1K desk, and they said that this was a known issue for such flights. However, the agent said she could apply the upgrade manually to the domestic segments. It took a few minutes, but it seems like you need to call United if you want the PlusPoint upgrades to apply for the domestic segments when you an international trip with Premium Plus segments.