For everyone else frustrated with this "feature" here is a workaround that might be helpful:

Create or update a budget to include at least an intentional $0 total for every month on every account that you always want to see in your report even if there are no transactions hitting that account in the period.

Then run your Profit and Loss report including that budget.  That should show every account even if there are transactions hitting those accounts in the period.  

At least that worked for me.