Go back and revisit your belt routing. I added "belt sides" info to the first image below.

There is no "adjustment" to affect the belt tension, tensioning is done by the spring that pulls the idler bracket to tense the belt.
There is however, what appears to be a "coupling join" (see inset in second image) in the PTO engaging linkage.
The parts IPL is "vague" about this junction but if this is an actual, separable join point, it is possible that a key or set screw may be missing, permitting the lower section of linkage to be "free" instead of holding solidly to tense the spring.
It is also possible that this "coupling" may have male & female irregular shape or sides to index the two halves........and the mating halves have "rounded out" from wear.

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