Free disk space is not accurate
Since macOS Sierra, Apple have chose to add the amount of truly free space to the amount of purgeable space, and that’s what is displayed on your Mac as the amount of free space on disk, in Finder informations, Disk Utility...
As you can see below, Final Cut Library Manager displays only truly free space:
148.21 GB - 3,94 GB = 144,27 GB.
That's fixed my problem!
That hasn't fixed my problem...
To complete, note that the Finder or the API that provides the information to FCLM, may take some time to give correct disk free space. Sometimes the Finder even may need to be restarted to be accurate.