I've found a recent bug to the new merchant system (just happened today, did not before). When clicking on the sign and selecting "I'd like to update my inventory from my token" the timer for the stall rental runs out very shortly thereafter, even when you've just rented it.
I can confirm there is an issue. Had my claim expire very quickly 3 times. Then I moved stalls and had no issues. 20 minutes later the original stall said your claim has ended, but no one was there and it was already expired. The stall in question is the second stall heading into Ticker square from the Open Door, the one that faces the Open Door.
But perhaps as Eraamion is saying it is something to do with revoking your claim is not removing a variable. Because that stall is the most commonly used.