My goal is to disable transparency for enemy nameplates that are not my target regardless they are in/out of range, in/out of combat, so basically all enemy nameplates would not have any opacity applied to it.

I'm pretty sure that this is achievable through any combination of available controls, unfortunately I was unable to locate the exact configuration options for this.

I'm using the following version of the addon v560.4

I'd argue you have some alpha mod active on your modding tab. Check there. Or try a default profile first.