Hrbrgr, first, thank you and Grantler for trying to help, and sorry for reacting to this as a substantial debate, and perhaps there is some misunderstanding. The reason I did not (close my dozen documents) and restart in Safe mode was because I did not see how this would be any different than a new installation, which I tried and had the same problem.
Anyway, I went ahead and started in Safe mode, and had the same experience in seeking to see my added saved custom gradient schemes so that they are listed after closing the saved ODT document and reopening it while still in Safe Mode (as well as closing LO in Safe Mode and relaunching it using the start menu entry LibreOffice (Safe Mode) on Windows, and opening said document, which it showed a recent document).
Attached is the file as a odt as well as a pdf in case the screen shots are not preserved in the former. Next, I will try this under a new Linux Mint install.
LOsafemodeGradientTest1.odt (260.6 KB)
LOsafemodeGradientTest1.pdf (139.3 KB)