The Lexham Hebrew Interlinear (LHI) at Isaiah 66:15 has an odd translation under the Hebrew word for "fire.".
I believe this is expected, if you hover over the translation there is a pop-up, this indicates its part of the notation. You can see the same type of notation in Isaiah 66:23 under the word for rebellion. Also if you add the Hebrew Lemma line in the internlinear settings it breaks the word down even more.
It is clearer if the Hebrew lemma line is included, but it reads "in the | a fire" i.e. in (the or a) fire. You should see an explanation if you hover over "the | a".
The appearance of the English will depend on your Default Resource Font.
Thank you. My old eyes misinterpreted the vertical line as a letter, not merely a separator.