So i've taken a better look into this and spoken to Mod Daze. What i said in my original comment was partially incorrect as i misunderstood some of the functionality of the farming master cape. The cape uses free supercompost (not from your storage/bought from Leprechaun) and should prompt you if you'd like to use ultracompost instead - however the latter isn't currently working.
As for your issue, it looks like you have to have a specific set of conditions for this to happen in that you must have:
certain composts stored
bought certain composts from the leprechaun
auto-composting for specific patches turned off
and wearing a master farming cape during any interaction with those specific patches
It only appears to be affecting ultra-compost. I've thrown in a bug for this and the ultracompost prompt (or lack thereof) so this should be fixed and out in game in the following weeks.
For now, it's best to get a refund from the Tool Leprechaun for any bought ultracompost, which you can buy again later for the same price.You can do this by talking to the Leprechaun and asking "Can you stop composting patches for me?" and you'll be refunded exactly how much you paid for the ultracompost refunded to you.
Hopefully that explains things a bit :)