Just been trying to google a bit more to see if this mirrors anyone else's experience, but I can't really find anything that matches the issue exactly.
I found one thread from last year where a buyer is talking about only seeing "buying options" on a listing, and no buy it now or add to cart, but the listing in question was actually out of stock and that's why there were no options to purchase, which I can see your listing still has stock available.
Just to eliminate it as a possible cause anyway, do you have the out of stock option enabled? And if you do, try disabling it (or even enabling then disabling it to check if it has any effect). I don't know how to get to the option in the app, but on the full version you go to My eBay > Account > Site Preferences > Selling Preferences (at the very top of the page), then there will be a check box for whether or not you want to use the out of stock option (having it set to yes means the listing doesn't end when sold out, but can't be purchased anymore unless you add new stock).
