there are likely multiple factors involved, and multiple reasons why online check in might fail. And likely not as black and white as you make it out to be.

Case in point: I frequently book travel to Canada on one way tickets. And have rarely had problems checking in in either direction. Sure, my ‘return’ ticket on a separate itinerary is almost always also UA, but I highly doubt One ticket purchase is connected to the other in order to see it. Saying an international O/W will always block checkin is just not correct.

there are also other items that can potentially cause check in to fail. Visas are a big one. My understanding is the UA app can scan a visa for China, but haven’t heard successful reports of other countries visas being scanned into the app to allow checkin - you get the same thing. I travel to India enough, and there is no way to scan an India travel doc into the system, so no matter what, you can not complete checkin online. Even though I have an OCI which is essentially a lifetime visa - I still need to show up at the airport to show it to them before I can get BPs.

try and relax a bit - just show up at the airport and have proof of your oneward ticket, and you’ll be just fine.