The clutch would certainly have to have been removed for the engine service to be performed and it is normal to replace the clutch using loctite on the taper shaft and the nut thread. I have known clutches spin on the shaft in the past following removal and re-fit even after the clutch was apparently correctly refitted so please do not blame the builder entirely as I am sure they would have believed the fitting to be secure. Unfortunately as you did not run the motor when you had it back you did not discover the problem and report it top the builder who I am sure would, have been very apologetic and refitted it without charge. The problem with the fiting of the Rotax clutch is that the back plate / ring gear is a press fit on a taper shaft and as such is prone to 'spin' on the crankshaft if the nut becomes even slightly loose.
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