I was working on a custom plugin in 8.5.3. It was working fine. Even new plugin generation, etc... all were smooth in 8.5.3.
I had tried the same on 8.6... and now in 8.9. In both these versions, I just generated a helloWorld plugin using the generate_plugin script. The plugins were generated but, both returned error on kibana browser load.
Refused to execute script from 'http://<ip>:5600/nzf/9007199254740991/bundles/plugin/hello/1.0.0/hello.plugin.js' because its MIME type ('application/json') is not executable, and strict MIME type checking is enabled.
Please refer to this link also (Kibana plugin development: Error when creating plugin in Kibana 8.6)