Not my experience -- recently flew AAA-ICN-BBB on DL/KL r/t and was able to select seats via the Delta app for both KL legs. Econ on way out and J on way back. Both seat assignments stuck for both myself and my travel partner (different tickets).

KL does love to change equipment though, so the nice AB seats we had on an A333 were changed to window middle on a 787 a couple days before departure. J seats stayed same (no equipment change).