We really need the model numbers off the two mower from the serial number tags. As when I tried looking up the mowers I came with three different transaxles being over the years and RS800 wasn't one of them. That don't mean the RS800 wasn't used, it just that I may have not found the correct IPL.
Also just because belt is new doesn't mean all the plastic idler pulleys are good. The V-idlers are especially prone the wearing allowing the belt to bottom out in the groove and effectively lengthening the belt. Also you need to install the correct length belt for the model you are working. As I said there is at least three different drive belts used depending which transaxle used.
2 mowers both models 944.604260
Mower 1 ser.#050314A001991
Mower 2 ser#042514A002139
Both use belt # 197253
Mower1 has limited reverse, mower 2 has no reverse, both belts are turning the transmission
Both mowers are Canadian
Review this service bulletin and see if this is your transaxle. If it is it contains information that may help you diagnose the problem. If not I’ll do some more checking. https://apache.ariens.com/manuals/B-2153.PDF
There is two drive belts. One from the engine to the transaxle and the one that actually is on the transaxle.
Now the 197253 (532197253) is listed as 1/2 x 100.97". This belt was superseded to 592855001 which is 1/2 x 100.25". Also I have seen both v-idler to wear out on this system causing belt slippage under load.