This software, installed from Mint's software manager, will not sort transactions in the ledgers by date. I have set each ledger to sort by the "post date" as the only item to sort on, as well as the first item in two (or more) and it still does not sort by date. As this is the best (and my only) way to sort ledgers not sorting by date makes this useless as it does not correctly calculate running balances if sorted on any other column and I cannot locate transactions quickly without a sort by date.
Two issues here. First, 5.1.3 is no longer supported, so please try 5.2.1. If it is not available for Mint, try an Appimage, snap, or flatpak. Even if you did find a bug in 5.1.3, it will not be fixed. Second, post date has always been (as far as I can remember) the first default sort item. What is your ledger sorted by? What do you see if you click on a column header in the ledger? Also, from what I remember, even in 5.1.3, if you sorted by anything other that date as the first sort item, it would not calculate a running balance at all, as you are correct about what happens.