Appears I had higher thoughts about your quality standards than you do , boskid, posila. :-D

A bug, by common definition, is when the function of a program deviates from the requirements.

As the internal names can be accessed through the rich text interface, they are part of the UI. Any name ...

now it's too late, as renames of internals breaks things (mods, blueprint strings, ...)

Kind of guessed that.

Me, I'm happy I actually found a bug in Factorio -- I have the highest respect for the quality software you churn out.

What did you do?
In a new game, upon entering the map with 'm' and then right clicking to add a tag with Name "[item=efficiency-module-3]",

What happened?
the text is marked red

What did you expect to happen instead? It might be obvious to you, but do it anyway!
rather than turned into the ...