I’ll start by saying that I’m using a CalDAV (Nextcloud) server as my sync backend, not Apple Reminders, so I know it’s possibly a niche case and might not be solvable. Which is fine, I’ve switched to using due dates as start dates and that has been fine.
When I turn on start dates, they work just fine locally but the start dates don’t sync between my iPhone and iPad. The rest of the task syncs, but the start date isn’t there.
I know that the Nextcloud server is capable of handling start dates because I used to use 2Do with it and 2Do was able to. However, I know that Goodtask goes through Apple’s Reminders system and Apple doesn’t seem to care about start dates, whereas 2Do went through CalDAV directly. As I said, might not be solvable, but I thought I’d raise the issue.