Seems this was fixed with the patch as well as there was no answer.
Going to mark this as fixed for now but leave open for a moment longer.
Please post if you still newly have this on a vanilla game.
Product: The Sims 4
Platform:Mac
Which language are you playing the game in? English
How often does the bug occur? Often (50% - 99%)
What is your current game version number? 1.78.58.1230
What expansions, game packs, and stuff packs do you have installed? All EP's except for Cottage Living; All GP's except for Dream Home Decorator, and Journey to Batuu; All SP's except for My First Pet Stuff, Kids Room Stuff, and Toddler Stuff; none of the Kits
Steps: How can we find the bug ourselves? Have at least one save file you have played in; in any of those saves have at least one pre-made Sim whose name you changed (can be from any pack); check their names in CAS
What happens when the bug occurs? instead of displaying the new name you gave them before this bug occurred the game will show their original name
What do you expect to see? the name I gave them
Have you installed any customization with the game, e.g. Custom Content or Mods? Not now. I've removed them.
Did this issue appear after a specific patch or change you made to your system? Neutral/Not Sure
I was updating a Sim I created using Morgyn Ember from Realm of Magic as a base. When I checked his name he had Morgyn's name instead of the one I gave him. I checked some other pre-made Sims I've changed. All of them had their original names again. To be more precise I changed Bjorn Bjergsen from Get Together and Cletus Harris from Eco Lifestyle. When saving them with the new name to my library and then loading them into a different save they kept the updated name. All save files I have created to this day are affected by this. I'm not quite sure when exactly this happened because the last time I interacted with any of those Sims was before the Dream Home Decorator patch. What's also interesting to note is that the names that were reverted back to their original ones use the English version even though I have started playing the game in a different language. So for example Bella Goth is Bella Goth again instead of Bella Grusel which is her name in the German version of the Sims 4.
