Fortunately, this is a relatively rare set of circumstances.
High Sierra has a number of issues in reporting free space on drives though they are generally cosmetic. This is gradually improving: it no longer tells me that my 4GB memory stick has 4.5GB free, as an example.
It's quite distressing because it was apparent early on (months before we had APFS even in Sierra) that APFS free space would be more complex to report to the user than HFS+ and some interest was shown in how macOS would do it. Sadly, it seems that the system designers didn't really know and High Sierra started life by reporting all sorts of nonsense (my 1 TB internal drive had about 4TB free at one point). When this muddled reporting extends into software it becomes quite dangerous.
Derek Wright 