Replacing a glyph with itself + other glyph

This works in Chrome and Edge:

<MultipleSubst index="0" Format="1">
  <Substitution in="g18" out="g10,g19"/>
</MultipleSubst>

But this only works in Chrome:

<MultipleSubst index="0" Format="1">
  <Substitution in="g18" out="g18,g19"/>
</MultipleSubst> 

I want g18 to be replaced by itself and a second glyph (g19 in the second example).

In Chrome this works, but Edge will not show the glyph if it's also the in glyph. This first example doesn't have the in glyph in the out sequence, and works as expected in both browsers.

I'm using GSUB Multiple Substitution Format 1. Should I be using something else, or is this a bug in either Chrome or Edge?