Okay, you all are going to think I've finally lost it. But this has happened to me enough times that I know it's real. More importantly, I'm certain it's not user error.
Reproducer steps: In Mail, open a message. Press Command+R to reply. Type the first sentence of your reply. End the sentence by pressing a period. Press the spacebar. Then type your second sentence.
Sometimes but not always (very rarely in fact, and not at all reproduceable), the spacebar following the period is ignored by Mail. The result: Your second sentence starts immediately after the period with no intervening space.
Anyone else? I mean, if at least one other MacOS user is seeing my solidus issue, then I figure there's got to be someone beside me who is experiencing the mysterious missing-space-after-first-period-in-email-replies issue.
It's likely, even probable, that this is VoiceOver related. I have "while typing speak" set to "characters and words", so I expect to hear "period" right when I press it. But, when typing replies in Sequoia Mail, I sometimes experience a slight delay before VoiceOver announces the period character. Whether these two issues are related or not, I don't know. But if VoiceOver has one problem with periods, it probably has others.
Yes, I opened a feedback report.