I have a GT2550 and so long as you have the correct belt , would not use aftermarket
The belt goes over the clutch V in then down to 2 pulleys with a 90% twist V in , across bottom of mower with another 90% twist thence over the deck drive spindle V in
Then tension belt using lever
If your belt is installed like that it should work properly , if it is installed correctly you must have a problem with the various deck spindles (blade and drive)
The break is it a snap or a cut type break ??
The following is the deck belt layout , I cannot find a schematic for the deckdrive belt
Oz
https://www.cubcadet.com/en_US/conte...5841d1d158a4/y