We recently ran into an issue trying to convert a color pdf file to greyscale. The resulting file when opened in adobe is corrupted, but what is shown still has some resulting color in it.
We have been able to reproduce the issue with your shared PDF document and Aspose.PDF for Java 20.1. We had asked for a resultant PDF document just to compare with the results on our side. Nevertheless, an issue has been logged in our issue tracking system as PDFJAVA-39177. We will further look into details of it and keep you posted with the status of its correction. Please be patient and spare us little time.