You might try starting SQL Developer from a Cmd window with the -clean parameter, as in
...\sqldeveloper\sqldeveloper\bin> .\sqldeveloper -clean
This will reset the cached user settings that are found in the system_cache, system_cache_1, ..., etc, folders (one for each instance you start) under the user settings folder for the SQL Developer version in use. For example, for the 19.1 release, under
Hope this helps
Yes - thank you Glen - that indeed fixes the issue. Even after rebooting, it now starts on the first incarnation without issue.
After starting once with the "-clean" arg, it starts reliably. Then "Windows" -> "Reset Windows to Factory Settings" to get the original window layout back. After that it seems to be completely fixed based on my testing so far.