Before you search for the css file, can you enable diagnostic features for the operator? You can enable this by navigating to the operator icon in designer studio and go to Preferences. After doing this you can search for the py-icons css file.
Also have a look at the UI gallery to test the icon display for a collapsible layout.
Adding UIKit didn't help. Even though the gif is present in Pega-WB ruleset which is in the ruleset stack of on of the Built-on applications it was either not getting picked or overridden in rule resolution. However when the gif was saved as in one of our own ruleset the issue got resolved.