The icon was changed from THUNAR_STOCK_TRASH_FULL ("user-trash-full") to "user-trash" in the Thunar code, and the commit author confirmed that this was done on purpose. https://git.xfce.org/xfce/thunar/commit/thunar/thunar-standard-view.c?id=728339f536ca03cde003d5c9320a8c3a4b627cae
Andre Miranda referenced this bugreport in commit d7d06a8efadbc34962e9742a8bb59b7804145b98 Make 'Move to Trash' icon the same as Thunar (Bug #15234) https://git.xfce.org/xfce/xfdesktop/commit?id=d7d06a8efadbc34962e9742a8bb59b7804145b98