I was looking at the code which implements opening walls in the JSW64 game engine.
The way the existing code works, the pixel-rows disappear from top to bottom, and then the attributes of the blocks are turned from Earth to Air. However, if you have a opening wall which is several blocks high, none of the blocks get turned to Air until the whole wall has had its pixels cleared. So you can temporarily have an INKless wall, several blocks high, before the whole lot turns to air.
Having scrutinised the pertinent code, and thinking out loud, I wonder if the original intention was for each block in turn (from the top one down) to turn to Air as the eight pixel-rows of each are cleared. A simple single POKE can be used to achieve this - and it involves reversing the conditionality of a relative jump (so a simple mistake by John Elliott when writing the code might explain the way the feature ended up, if my hunch is right that they were intended to disappear one-by-one).
Have a look at the two test files attached. They illustrate the difference between the two variants of opening wall mechanics. The opening wall in the 'unfixed' file behaves exactly the same as in the unpatched JSW64 game engine. In the 'fixed' file, I've changed the value of address #FFD9 from #20 (JR NZ) to #28 (JR Z).
Willy starts off in the Off Licence. Collecting the single item in the room causes the wall to start opening. I've come up with a little challenge based on the different behaviour of the opening wall - only in one of this pair of files is it possible for Willy to leave the room and access The Bridge.
EDIT: Both files have been updated, with the addition of an extra Fire cell to prevent an (unintended) loophole.