Household Sims loose existing relationship

Could you please edit your post and add the following info to your post.

Product: The Sims 4
Platform:
Which language are you playing the game in? 
How often does the bug occur? 
What is your current game version number? 
What expansions, game packs, and stuff packs do you have installed? 
Steps: How can we find the bug ourselves? 
What happens when the bug occurs? 
What do you expect to see? 
Have you installed any customization with the game, e.g. Custom Content or Mods?
Did this issue appear after a specific patch or change you made to your system? 

The bug template is sometimes not being displayed due to high traffic.

I went back into the recovered save and tried to do the same thing, which was make two cakes, invite the teens' significant others to age them up too, age up my two teen sims, and then get one of them engaged to his girlfriend, but I experienced different problems instead. One of the sims I invited over said yes but then had to be invited again before he showed up, and then he didn't come onto the lot until I called him over (this has happened before with that same sim, but only since the Lovestruck update). Both he and the other sim I invited over could not be aged up using "Age Up" on the birthday cakes, when this worked fine the first time I did this. I could only age up random townies that were in the neighborhood at that time. I sent one of the sims I invited over home and tried to invite him back and he said yes but never appeared. The other sim I invited over did not have the option to be sent home. 

I did not experience the same glitch as before where the parent sims became strangers to each other, but I don't know if what triggered that was the other sim getting engaged which I couldn't do because his girlfriend couldn't be aged up. I also didn't save and just exited to the main menu after I had the two new issues. Should I go back into the recovered save to try again?

I do want to add that I have been playing this household since last summer, but I've saved as multiple times and this current save file is only from June 2024. I'm planning to save as again in a few days. 

@AnneSims34 I wonder if this is a fallout from the issue where you can't add third traits to young adults when they age up from teens.

A LE occurs with that error and often if one occurs, the game abandons the rest of the things it's supposed to do.

Did you have any issues with that at all ?

In the original save I was able to give both the sims in my household I aged up their third traits, and their significant others also got third traits when I aged them up. I've been playing the same save since I had the original issue, and it's been normal since (fingers crossed!), just a little laggy. I have had a couple teen sims in the save (not in my household) that never aged into young adults, and couldn't be aged up using the cake, so I just deleted them, but that's the only major issue I've noticed with teen sims in this save. 

What is an LE, just so I can be on the lookout?

Thank you!

Today I opened this save again (though I have saved as since the first time I had an issue) and I have the same married couple who lost all relationship before still in my household. They had lost all relationship again, which I didn't notice until I tried to make them go to bed because they wouldn't sleep in the same bed (this was not a routing issue as even with the bed in the middle of a large room they wouldn't sleep in it at the same time). This time I got no notification that they lost their best friendship status, their relationship was just gone when I opened the save. So the same couple has lost all relationship twice now. The only other relationship changes in the household happened with another sim, who broke up with someone and entered a relationship with someone else, and I also moved a sim into the household. I did have that sim ask about their partner's romantic boundaries, I don't know if that is contributing to issues?

Hi sorry I didn't see this until now and those sims that were having the relationship disappearing issue died but I did have it happen I think at least one more time between two of my sims in the same family, save, and household who are siblings. Would it still be worth it to send the save file?

Also I saw the comment from crinrict theorizing that it might be related to sims aging up to young adults and not getting the prompt to pick a third trait and that did just happen to me in the same save with a grandchild of the two original sims to have their relationship disappear. Is there a fix for that problem? I was just gonna go into CAS and give him a third trait manually but I didn't want to potentially screw up my save so I just exited the game. Also that sim is dating the grocery store delivery girl and they just adopted a cat but I don't know if either of those has anything to do with the issue.