I’ve seen similar issues when building a JUCE plugin. A first (debug) build using High Sierra would not scan in auval (and hence not show up in Logic Pro X) until I log out or restart the mac. Funnily enough, despite the file being visible in Finder and in the terminal, auval claims that the plugin does not exist. After a reboot, auval does ‘see’ the plugin and validates it successfully, after which it shows up in Logic Pro as well.
Using the exact same project and process this happens on one machine but not on another one. Very confusing. The same project compiles perfectly fine on the same two macs when running Sierra. Note that High Sierra did upgrade the file system to AFSP.
I have found one work around for my case when building plugins. If I rename the AU plugin from its original name (let’s say juce_plugin.component) into something else (juce_plugin2.component), and then rename it back to its original file name (juce_plugin.component), auval scanning works fine and the plugin shows up in Logic Pro as well.
Don’t ask me why this works… :-/ The issue seems to be with auval claiming the file is not there until it is renamed, the user logs out, or the mac is rebooted.