Yes, sorry, I meant an older version of pyxform, the library that converts XLSForms into a format Collect and Enketo can use. We maintain pyxform and typically keep it up-to-date in Central. Kobo is currently still using an older version that predates the pyxform change I mentioned.
I've confirmed that that change was indeed what resulted in the error that you see. I do think the structure you've ended up with where the choice-name calls are in the repeat is better. But we also try not to break forms that used to work if at all possible. I have put in a change that will make the old structure work and we'll try to get that released soon.
Did you see an example of the type of expression that you tried to use somewhere? I'd like to change any documentation or examples we have that might show it.
Ah yes, I see. This is an XLSForm limitation, currently. We detect ${} and instance expressions but expressions of this shape are a little trickier to identify correctly. I've filed an issue at https://github.com/XLSForm/pyxform/issues/703