Try disabling the built-in android backup system? I'm just guessing here though.

I've taken to DIY-ing things like this using Tasker simply because it's the most reliable long-term solution: whatever trigger I want with whatever notification options I want and it never stops working. Plus, triggers can be dynamic and notifications can be set to be undismissible by swiping¹.

What I'd recommend that you try (as written, you only have to do the Tasker setup once and it'll work for every event you do the calendar setup for):

• (calendar side) set up a calendar event for what you want a task for, have the title start with a prefix of some sort (short, and ideally one that won't occur naturally, so something like [tsk] (note the lack of vowel: ensures that it can't be matched accidentally by an actual calendar event. Ditto for the square brackets))
• (calendar side) make sure it sends a notification for when you want to be reminded. Pretty sure that you are forced to use the 10 min notification option that's there by default (I've never managed to remove or change that one, but I digress) so add another if you need to.
• (tasker side from this step onwards) create a new named task that uses the AutoNotification plugin to dismiss the calendar event notification with that prefix, then switch over to the profiles list (we'll come back to this)
• create a new event profile using the AutoNotification plugin and configure it to intercept every notification with that prefix in the title and from the Calendar app (by matching on the app in addition to the prefix, you don't need to figure out how to remove the prefix from the title later on. I mean, you can, but you don't have to)
• when it asks for a task to use, choose the one you created earlier
• go back to the task you created, then start having fun!

From there, you could have tasker post a notification with the event's title, configured as persistent, with a button in it to dismiss it. Or make a long beeping sound. Or open an app. Or etc etc.

¹ aka "persistent", but that's a bit ambiguous in context. What does persistence mean here? Does it come back after swiping it away (you could do that, but it's extra work)? can you swipe it away? Etc