If you log into your Skylum account, you will see, that none of the Skylum photo editing extensions (Luminar 3, Aurora 2019, Luminar Flex, Photolemur) can currently work well with Photos 5.0 on Mojave.
The recommended Fix is to launch Luminar 3 with Image > Edit in Luminar 3.
- Now edit the photo, then save it back from Luminar to Photos with "File > Open in Photos".
This is currently the only way how I can use Luminar at all under Catalina.
It is not a Catalina Problem. The Skylum Team needs to get their apps ready for Catalina. I kept one Mac running Mojave, so I can still work with the applications, that cannot yet be updated for compatibility with Photos 5.0 and Catalina.