@Phoenix616 is using the Windows version of the launcher.
From their comment:
Used launcher version: Windows 10.0 2.3.508.
This seems to be an issue specifically with the beta version of the launcher.
The standard release (at least on macOS with version 2.3.462) seems to behave correctly.
Phoenix616's repro steps will also repro the issue on the beta macOS installer.
Moreover, the launcher refuses to save any resolution changes at all, even when creating a "new installation" entry.
Thanks. I did search and didn't find the duplicate.
Beta 2.3.520 addresses the issue for me. Thanks