In 2.05, it was decided to add a frobbox_size of 10 to purses. See issue #4150.
As was just discovered (issue #4573) this prevents a purse in The Creeps from being frobable.
The purse isn't frobable because the increased frobbox_size allows the frob controller in the drawer under the purse to think the purse is in the drawer. So when the drawer is closed, the purse is unfrobable.
Since the purse has been assigned to the drawer, it isn't assigned to the secret door. When the door is opened, the purse is ignored and it doesn't become frobable.
My take on the discussion here is that folks thought it would be nice if purses automatically received a bigger frobbox_size to make it easier for them to be frobbed out of chests.
But the bureau is a complex beast full of frob blockers and frob controllers and increasing frobbox sizes amongst that complexity causes problems.
I propose reverting the 2.05 +10 frobbox_size on purses and going back to "it's up to the mapper to make sure items inside chests and bureaus and desks are frobable", which is how it's been since day 1.