When trying to select multiple objects with the same fill color and opacity, selecting one of them and under the Select menu choosing Same > Appearance will add all the same fill and opacity objects to the selection.

However, when creating an object via an action and setting the fill color and opacity to match the objects to be selected, Select > Same > Appearance returns no results.

Manually selecting multiple objects in addition to the object created with the action shows "Mixed appearances" in the Properties panel. Individually selecting each of the objects shows the same attributes. Selecting multiples of manually created objects all show the same appearance properties. Adding an object with the same attributes but which was created via an action shows "mixed appearances."

This was posted in the Adobe community forum: https://community.adobe.com/t5/illustrator-discussions/select-same-gt-appearance-works-manually-but-not-in-an-action/td-p/14898865

Here are the steps to repeat:

Make a new, blank document.
Start recording an action in the Action panel.
Record yourself making a new rectangle with the Rectangle tool.
Set the Fill to Black, and the Stroke to None.
Change the Opacity in the Transparency panel to 20%.
Stop recording.
Move that frame to the side.
Manually create a new rectangle, using the same steps as above.
Under the Select menu, choose "Same > Appearance."
You should see the previous rectangle you made included in the selection.
Your Properties and Attributes panels should contain the same information for both objects.
Now run your action you just recorded in the Actions panel.
Choose Select > Same > Appearance.
Note that nothing else gets selected.
Manually select the rectangle created by the action plus either of the ones created manually.
Note the Attributes panel now says "Mixed Attributes."

You recorded yourself making an object with certain attributes. You played back that action and according to every panel it has the same attributes as the one you previously created, but you cannot use Select > Same > Appearance to select them both because the appearances are "Mixed."

The only way to work around this is to create a Graphics Style with the attributes (opacity, fill) being searched for. Then an object can be created with an action, the action can assign the Graphics Style, and then Select > Same > Appearance will find all of the objects with the same fill and opacity (even if those objects do not have the Graphics Style applied).