Opened Mechanized Airlock acts as a solid floor

This is intended behavior. Think of opened airlocks as similar to ladders- they can be walked on, but aren't a solid tile.

Note, however, that a critter will be able to fall through an open airlock (or a closed pneumatic door for that matter), if they are already falling.

Here is an example of this property that I take advantage of in my saturn critter trap farm:

When a pneumatic door closes on a critter, they will start falling- if the mechanical airlock below this hatchling were open (toggled via a switch), it would continue to fall past the airlock. Since it's closed, the hatchling will instead land on it.

Here's another example of how you can cause critters to fall:

Note how the drop-offs are positioned- critters will always move to the tile to the left of the drop-off if it is a valid tile for them to stand on, when they are auto-wrangled. After they have been trussed, they will begin to fall. In this case, this causes them to fall through the pneumatic doors. You can also see another critter dropper towards the bottom- the basic idea behind those is that the weight plate opens the top door so that the critter can move into the now open door's space after they hatch, at which point the door closes, causing them to fall. You can utilize this in various ways to help automate ranch restocking.

One final thing to note that I have observed: when using mechanical airlocks to control whether or not a falling critter will land on that floor or fall through it to the next, slicksters are an anomaly in that they are capable of landing on opened airlocks. I don't really know why this is.