I had this too for a long time, @Ed_Wegryn. More than once. The total number and the number at which it stopped both charged. But I could not get them reliably to complete.
I got used to the workarounds which @DDPS outlines.
Also restarting the Roon server.
In the end - at the direction of Roon support - I re-imported a corrupt track which they detected.
And have never looked back 
Reluctant though you may be to put this down to a track which - for whatever reason - Roon can’t consistently process, I strongly suggest that’s a worthwhile path to follow. Good luck!