The reason for the issue is that the lighting fixture family needs to have a "Save" (file) before making any changes to the Light Source Definition dialog box. If there is no that previous "Save", even if you specify the IES file for the light source, the shape doesn't update. I understand this as Revit needing to make a proper association between the latest version of the family and the external IES file being referenced. Attached is a Revit project with a lighting fixture with 3 types, each of them displaying correctly the correspondent shape of the light distribution, as per the IES file.

I'm not certain I understand exactly what you are saying, so let me repeat and see if I understand correctly:

You are saying that the IES web is not displaying correctly because the file is most likely still named "Untitled.rfa" because I have not saved?

-or-

You are saying that in order to update the photometric web, I have to save the file ater updating the IES options?

Either way, the issue that I was having would not fit either of these symptoms. Even IF the web did update, it would go back to generic if the light source plane was moved. Once it went back to generic, it would render with the correct patterns, even when the generic web shows.

After saving the file, closing it and reopening, the light source would still be displaying the generic web even though it would render the proper IES pattern.

Never resolved this because it was rendering ok, just had to guess on my orientation.

Well, the pattern I found is that I need to click on the Save button before making any changes to the Light Source Definition dialog box (to select Photometric). Then, after that, if I select a IES file, it displays correctly. Otherwise, it fails.

It's as if in this workflow:
Specifying an IES File for a Light Source - WikiHelp
...we were missing a step between # 3 and # 4. Let's call it #3.5 = "Save the family file".

I'm still no sure why the photoweb would switch back to generic after moving the light source. Here's the workflow when this happens:

-Open Custom Light family with IES already specified that was previously closed with photoweb displaying correctly.
-Photoweb displays correctly.
-Move light source elevation up/down
-Photoweb switches to generic, no longer displays correctly.

That's it. It's buggy, not sure if anyone's had this experience or not.

I had a similar issue once. Curiously enough a reboot of windows!! cured that behaviour.

-rpict