EDIT: Changed the name of the post to better reflect the nature of the bug.
While a manual airlock is closed, it will not provide a decor penalty to any tiles around it. The bug persists even if the door is temporarily opened by a Duplicant walking through. The bug also persists if the airlock is opened permanently. If the airlock is open when the save is loaded, however, the door will provide a decor penalty until it is closed, after which it will provide no penalty even if reopened.
What is shown in the decor overlay is consistent with what is shown in a nearby Duplicant's Status tab.
I would guess that this bug is due to there being no unobstructed line of sight to whatever the "origin" of the decor is because the airlock itself blocks it.
It seems very likely that this bug is related to a bug I posted previously about entombed buildings losing their decor value until the save is reloaded.
I have not tested this with Mechanized Airlocks, but I have noticed that they do not provide a decor penalty when closed either.
If the airlock is closed when the save is loaded and then opened temporarily by a Duplicant passing through on "Auto" mode or opened permanently by a Duplicant setting its mode to "Open", it will provide no decor penalty. (shown below)
If the airlock is open when the save is loaded (either by a Duplicant passing through or by being set to "Open"), it will provide a decor penalty. This behavior persists until the airlock is closed. Once closed, it will again stop providing a decor penalty, even when reopened. (For reference, the decor radius of a manual airlock is 3 tiles, so it should contribute a penalty in the image shown below)
Steps to Reproduce
Build a Manual Airlock. It will not provide a decor penalty, even if temporarily opened, if it was closed when the save was loaded. If it was open when the save was loaded, it will provide a decor penalty until the next time it is closed, after which it will no longer provide a penalty even if it is reopened.
This issue has not been confirmed by a developer yet.