Tried to load a plugin's code, but failed. path=>"logstash/filters/<filter_name>"

Hello,
I'm trying to use my filter plugin in a conf file. It's being properly installed via an offline plugin pack, but errors when running logstash:

ERROR registry:123 - Tried to load a plugin's code, but failed. {:exception=>#<LoadError: no such file to load -- logstash/filters/pubsub-mysql>, :path=>"logstash/filters/pubsub-mysql", :type=>"filter", :name=>"pubsub-mysql"}

I've tried adding a path in Gemfile (vendor/local_gems/...), but it didn't help.
What could be the issue here?

Thanks