Custom plugin's old source code gets executed

I’m developing a custom plugin and have an issue: I observe that occasionally an older version of the plugin gets called (I see old debug log records that have been removed in newer versions).
Once installed (globally, not bound to any routes, etc) I’ve been modifying the handler.lua file only. After each plugin source code modification I restart Kong.
This picking up of different plugin versions (the old and the new ones) appears to happen sporadically withing the same Kong run.
Are there any ideas why this can be happening?

Thank you.