The Balance Sheet Comparison report will compare the same months for the current and previous year. I can share a workaround to get around this.
You can export the said report to Excel instead and then create your comparison for the current month (YTD) and December of last month (Previous year). Let me guide you how:
Go to the Reports menu at the left pane.
Look for and select the said report from the search menu.
Click the export icon, and then select Export to Excel.
Then, make the necessary changes from there.
Also, I'd agree that having the said filter can make the comparison easier and this looks like a good addition to the program. Thus, I recommend visiting our QuickBooks Blog page to get you in the loop about the recent news and product enhancements. All details are laid down there so you can check it from time to time.
If you want to filter the said report further, you can customize it. When you're ready, you can memorize it to save a copy and access it quickly for future use.
I'm always around to lend a hand if you have follow-up questions about managing your reports in QuickBooks Online. I'll be here to help you some more. Take care.