Issue was two parts. First, example of nested language included a case not handled correctly by the tree-sitter parser. Second, injection was set at lower scope for other cases.
Leaving original question in case this helps others.
In playing with the new tree-sitter setup I’ve managed to get things like highlighting nested languages with injection working without any issues. I have noticed however as I’ve been adding folds that injected/nested languages aren’t showing their subsequent fold options. If I apply the syntax in a separate file they’re recognized just fine, but again not when nested/injected. Is there a particular instruction or caveat necessary for folding when injected?