Prologue - issue with SR parameters

I was trying to create some sound with the Prologue synthesizer, and noticed there was some kind of high frequency distortion I couldn’t get rid off. Eventually I discovered it was the SR (sample rate) parameters in the ‘EFX’ section. It was set to 1/2 and disappeared when set to full. When set to 1/4, the high frequency noise shifted to the higher frequency spectrum and was only noticeable when low notes where played but clearly seen with a scope plugin. This effect is best noticeable when a simple continuous sine is played without effects, otherwise the noise could be masked.

If I read the documentation, it should lower the sample rate and thus reduce the high frequency content. It cannot be filtered out with Prologues lowpass filters, so I guess something is wrong with the upsampling algorithm in the final stage.

Cubase 11 v.11,0,0
Windows 10 Pro 20H2 64-bit
AMD Ryzen 9 5900X 12-Core Processor 3.70 GHz
2 x 32GB RAM