Text is not filling until bottom margin

I’m working on Urdu Persian text, while typesetting on some pages the text is not filling available space at the bottom. Please see the screenshot. Can someone please help me to solve the issue.
This issue may be similar to Unable to fill text until bottom margin · Issue #79 · sillsdev/ptx2pdf · GitHub

My best guess is that the problem is to do with balancing columns. (I.e. Making sure that there are the same number of lines on each side)
I believe that at the moment the code has a ‘balance at any cost’ as approach. Thus if you have 5 lines in the next paragraph as I think I see, the code cannot find a good place to break it. 5 lines means:
3 lines on one side 2 on the other (unbalanced, and so bad)
2lines on the next page 1 line in one side, 2 on the other (equally unbalanced)
3 lines on the next page, 1 line on each side. (May be allowed, but not if the 2nd column starts a new paragraph, as breaking a paragraph after just one line is usually a forbidden thing.)

If you pick a longish paragraph that is on the page and use the adjust list to make it one line longer, then that might solve the issue.
Alternatively you may want to allow “orphan” / “widow” paragraphs.