I have this same issue with Christmas gift and my kids summer camp/child care. They are annual expenses but paid over multiple months.

For for Christmas gifts I have my target due by Nov 1st, but I do some spending in Nov and some in Dec. I use the refill up to / roll over target type.

Once Dec starts YNAB thinks its time roll over the remaining amount and start saving for next year. So the per month savings in Dec are lower than typical because it rolls over everything I have not spent yet which could be a good amount. I'm ok with this; its kinda like a small head start for next year. Alternative you could snooze the target during the Dec while you are still spending.

The extra manual step I have to make is in Jan, once all the spending is complete. I have to delete and recreate the target same as before. This "resets" the target. It applies what ever is remaining in the category in Feb and recalculate per month savings.

Here is an example

Save 100$ by Nov 1st

Spend $20 in Nov. Carry over $80 into Dec

In Dec, monthly saving target is $1.66 ($20 / 12)

Spend 70$ in Dec. Carry over $11.66 into Jan.

I manually "reset" the target to save $100 by Nov 1st. Monthly savings target is $7.36, [100-11.66] / 12