I've explained your problem, and the solution, in reply #5 ... Read it again !
Here is the output of your original code, when printing in the Serial monitor instead of your display
...
5:25:77
5:27:52
5:29:0-10
5:31:0-1
5:33:0-9
...
Here is the output of your modified code using sprintf, when printing in the Serial monitor instead of your display
...
5:25:33
5:27:07
5:28:-55
5:30:-64
5:32:-72
...
Why you don't see the minus '-' character on your display, you may ask ? Well this is the font that you are using MD_Parola_Fonts/SmallDigits.h at master · maxbanton/MD_Parola_Fonts · GitHub and as you can see, this character is not defined, that's why it's not displayed.