This is tangentially related to an issue already identified (inability to select seats on individual segments of a multiple-segment flight), but is more basic, and I didn't see it mentioned when I searched...

For about the last year of so, I've been completely unable to select seats on flights 92 and 93 (GIG-GRU-IAH). I can select a seat, but when I click on "Continue" the original seat assignment remains.

Even when I go to expertflyer and check for a seat that's available on both segments, it still doesn't take. I have to call an agent and have the seat assignment changed manually.

Seats for on-going connecting flights can be changed normally.