Why does the contraction of "I will" sound strange in certain sentence constructions?

Recently, while chatting with a friend via text, my friend asked me, "Can you ask them tomorrow?"

I responded with:

I will when I go.

It occurred to me when writing this response that it would be really weird sounding to say:

I'll when I go.

However, it would have been normal sounding if I had said:

I'll ask when I go.

Is there any formal reason why the version without "ask" following "I'll" is incorrect? Or maybe it is correct, but merely awkward?

I suppose there may not be any explanation deeper than, "it just sounds weird," but any informative insights into why, ideally with examples of similar cases, would stand as an answer.