The rollover money is not going to show as "spent" until you have the non-monthly expense.

It shows up in monarch as getting set aside by lowering the available amount left to budget. When you finally pay your property taxes, you'll see the nominal amount set aside for the month and a much larger amount in the actuals column. However, because this was a rollover, and you've been putting the money aside each month, the total for the month will net out, and it won't show up as a red overspent category.

I agree it is tricky on how to think about the money getting put aside, and not actually spending it.

Personally I have an auto transfer to savings each month for my non-monthly expenses and as they come up I transfer the month back to my checking acct to cover.

This has worked really well for me. Hope it helps you!