My guess is that the problem originates here - I think these flags will apply to all targets (not just the plugin), so the manifest generator will be built with the hardened runtime. This fails because apps using the hardened runtime can’t load adhoc-signed bundles, and the bundle isn’t release-signed until after the manifest is generated.
I recommend instead just setting OTHER_CODE_SIGN_FLAGS=--timestamp, and setting “Use Hardened Runtime” to “Enabled” in the Xcode (macOS) exporter in the Projucer. The Projucer knows that when the option is set here, it should only apply to the final plugin binary, rather than to every target in the project.