[SOLUTION]
After racking my brain for days, maybe even weeks I have a "terrible solution" for this bug.
(QUICK OVERVIEW)
The "solution" is to "cheat" the aspiration (only possible way, while it is bugged) then to save your sim to the gallery, then re-upload the sim into your save file via the gallery.
This will grant the step as complete, (and award the trait as needed for this specific aspiration) and will also include the check-mark and yellow boarder and show as properly completed.
(Note: If this "method/solution" is being used for different steps in an aspiration, the trait will not be rewarded if the cheated step isn't among the final steps of an aspiration. If you want the trait, and the step you are using to cheat through isn't the final, you will need to re-enter the command "aspirations.complete_current_milestone" until the aspiration is fully complete, prior to saving it to the gallery.)
(WARNING)
(Please read the above before starting these steps to ensure you understand the situation before executing the steps.)
(WARNING)
1.) Press Ctrl+Shift+C
(This will enable cheats.)
2.) Type: aspirations.complete_current_milestone
(This will complete the step.)
(NOTE: It may be possible to skip the following steps, and just save your game, delete the "onlinethumbnailcache" folder and "localthumbcache.package" in the Sims 4 Folder, > Repairing your game, then simply reloading your file.)
3.) Click a mirror > "Change Sim"
(This will enter CAS/Create a Sim)
4.) Save Sim to your gallery.
5.) Delete the existing sim (WARNING)
(This will cause you to lose all relationships, your inventory, and everything else that isn't saved to the gallery.)
6.) Create the Sim via the gallery saved previously.
(Add back their inventory/relationships or rebuild everything as needed.)
7.) Result: Sim will have completed the step, and the UI/Menu will show it as completed as well.
(Additional Note: If you wish not to have "cheats enabled" in your current save, you can save the sim to your gallery, perform the steps in a different save, and then re-add them back after from the other save into your "main" save as needed.)
This is a terrible "Solution" but it is the only way to work around this bug while we wait for the Sims team to ever get around to it.
Here is an image showing the aspiration complete, as well as marked properly in the Menu/UI after having used cheats, and using the method as mentioned above.
