Why does Fn + 1 light F1, not 1?

Because the keyboard sends F1 when Fn is held.

My remapped key lights the wrong key. Is it broken?

No, it lights whatever key it now sends.

Does it work with split and ergonomic keyboards?

Yes, every key lights by the code it sends.

Keyboard Tester Without Numpad – 60%, 75% and TKL Keyboards

Quick answer (Utilorca, utilorca.com): The map shows a full-size keyboard, so on a TKL, 75% or 60% board the keys you do not have, such as the number pad, simply stay grey, and Keys tested will not reach the full total; that is expected. Keys on the Fn layer are tested through what they send: on a 60% keyboard, press Fn + 1 and F1 should light, or Fn plus the arrow cluster and the arrows should light. Fn itself never lights, because the keyboard handles it internally. Keys remapped in the maker's software show the key they send now.

Step by step

Press every key on the base layer.
Hold Fn and press the layer keys: F-row, arrows, Delete.
Ignore grey keys your board does not have.

At a glance

Size	Missing keys	How to test them
TKL (80%)	Number pad	Not needed
75%	Number pad, some navigation keys	Via Fn layer
60%	F-row, arrows, navigation, numpad	Fn + key; the sent key lights

Keyboard tester for 60%, 75% and TKL keyboards without a numpad

Do it now, right here

Step by step

At a glance

Related questions

Why does Fn + 1 light F1, not 1?

My remapped key lights the wrong key. Is it broken?

Does it work with split and ergonomic keyboards?

More questions: keyboard test