I also have been trying to find an answer to this very question for over a year now.  The best explanation that I've been able to find is from a YouTube video from QuickBooks Made Easy by Gregg Bossen Check from a Prior Year in QuickBooksQuickTips™:Voiding Checks from Prior Periods by QuickBooks® Made Easy™  

According to this video and other replies that I've seen in several threads in QuickBooks Support and Community there should be a QuickBooks pop-up window that appears after you click Void on what I call the 'check screen' that reads To maintain the accuracy of your financial reports and balance the accounts affected by the check, QuickBooks can create a journal entry in the earlier period and a reversing journal entry in the current period.  Would you like QuickBooks to void the check and enter the appropriate journal entries for you?  Yes - No, just void the check - Cancel  You should click "Yes"

Anyway, this popup screen does NOT appear on my screen when I void any check and no one has been able to tell me why.  This same video shows what happens when the journal entries are made.  These JEs are not made automatically when using QB Online so I believe he explains how to make these when using Online in my case I have to manually make the JEs because of no option for me to click Yes and have QB Desktop automatically do these for me.