The problem is that requirements refinement has been unceremoniously dumped in your lap. The failure here is organizational; maybe you have a design person involved, maybe devs are expected to do this. Either way, your job now also includes communications.

One strategy I've used is to draw a low-fi example of what they're going to get - Figma is great at this these days. Then I add it to the issue and push the whole thing back for early approval in order to suss out these finer points.

Not to come off as misanthropic here, but many people are hot garbage at describing what's in their head. Most of the time, it's all abstract concepts up there until you start asking the real questions. They really do need a whole-ass conversation to sharpen that mental image. Or in this case, what they want that feature to look like. Incidentally, this is also the reason why therapy is a thing, and why it takes people years to make sense of themselves, and that outcome is usually far more crucial than anything we're doing at the keyboard.