I'm having the same problem as the person who asked this question: I cleared a bunch of files out of my Trash and instead of getting free space on the volume, I got 30 gigs of "purgeable space". According to the accepted answer to that question, I can engage in an entirely reprehensible workaround to liberate this space from its ghostly, meaningless existence in purgatory—a strategy to which I may well be forced to resort.
None of the answers to that question, however, deal with the more fundamental issue of why this is happening in the first place. Ideally I'd like to avoid having to do this ever again. Is there some way to tell the filesystem not to mark files as "purgeable" rather than just removing them? I'm running High Sierra, and the system drive (the drive in question) is using APFS.