After 34 years in the Macintosh environment (never used Windows), reading about Linux for ~15 years (wanting to get into it but slowed by chronic illness), I'm finally starting to learn about it. So I've installed Ubuntu 20.04 LTS in a partition on my second MacBook Pro (13" 2014, 11,1), alongside several versions of macOS (using 10.12 Sierra as my primary).
I've also installed rEFInd, started learning about it, but ran into some trouble, so removed it.
I know I can switch between macOS and Ubuntu by pressing ⌥ option during startup, then selecting whichever I want. For now I'm still mostly working in macOS, so have it set in Startup Disk Preference Pane as the default startup.
However, if I start up in Ubuntu, then restart or shut down and start again, it defaults to Ubuntu. (In rEFInd terminology, a "boot coup".) I've experimented, repeatedly resetting macOS in Startup Disk, but every time I start Ubuntu, it takes over the default spot. (I'm not sure if it was doing this before I installed rEFInd, or started doing it only after I began using rEFInd. Anyway it's continued after I deleted rEFInd.)
Is there some way I can tell Ubuntu not to do this?