I notice that in the markdown editor, if I type a double space rapidly, a period is inserted, if I type those spaces more deliberately, it is not. So it would appear that this is the intended behavior, but it seems intrusive to me: if I want a period, I'll type one. Is there, perhaps, a way to turn this behavior off?
I don't know about MacOS, but in iOS and iPadOS that's intended behaviour. It can be turned off in the settings section of the keyboard app, and there should be something similar for MacOS.
I agree wholeheartedly: "there should be something similar for MacOS"; but, how to control the MacOS equivalent of a "keyboard app" within the context of the Joplin application for the moment eludes me.