Thanks for the version details, @Rusti. I will try to take a look soon. In the mean time, the shortened CalcTest form would be helpful to see but it looks like a broken link now.
I've never used jr:choice-name because, as @martijnr mentioned, it feels like a hack. I can't see your form but what I'd try is explicitly building the XPath query for the choice name with something like jr:itext(instance('your_choice_list')/root/item[name=${MeshSize_ID}]/itextId). The catch is that this will only work if you have a choice filter on your choice list. You can read more about that and how to force it at Reverse cascading choice - #2 by LN.