OK, this is definitely a bug - I can replicate it and I know how to work around it.
1. The Adobe Premiere Elements (APE) window is available in OBS during the APE splash screen.
2. After you have selected a Video project in the APE splash screen, the APE window is no longer available in the OBS window list.
3. If you tick the "Show Windows with empty names" checkbox, the APE window pops up in the list.
4. Untick the "Show Windows with empty names" checkbox and the APE window remains in the list.
5. Remove the add a new Window capture source and the APE window is not listed. Repeat steps 3 and 4 to resolve.