Entering and exiting CAS or loading into a game removes Secret Lover relationship from replaces it with Partner/Dating/Boyfriend&Girlfriend status.
I first encountered this bug while playing with mods. So, I initially thought it was a mod problem. Specifically, EPO Mod 1: Family Preferences. I got the option to change my Sims' Preferences from "Not Wanting Kids" -> "Happy to have kids". When I clicked agree to the change, both of my Sims' Secret Lovers became Partners. I got the notification pop-up right after. They still have the sentiment but the "Secret Lovers" relationship tag is changed to Partners. You cannot ask to be Secret Lovers again without cheats/mods. It removes the interaction.
However, I encountered a similar issue with my married sims. I raised this concern in the this bug thread. With the married couple, after entering CAS (with cas.fulleditmode), made some adjustment to their traits, and returning to Live Mode, I got a pop-up that so-and-so became the girlfriend/partner of my sim. They just got married the day before. You have to re-enter CAS and manually set the relationship to Married to fix the bug.
I encountered the bug a THIRD time with Secretlovers again. This time, I entered CAS without cas.fulleditmode and without adjusting anything on my Sim (Simply entering and exiting), and my Secret Lovers (My Sim had two) were all changed to girlfriend. I got a pop-up (so-and-so became My Sim's partners etc.).
The issue is not limited to only CAS or Traits changing. It happens upon loading into a game too. I was testing WITHOUT mods and it did not take me long to re-encounter the bug.
First, I removed my mods and tested the game unmodded. As soon as I loaded into the game, this happened:
- My Sims spouse remained married (good)
- My Sims' Secret Lover became a partner. I received a pop-up of the relationship change.
- My Sims' high romance (but not Girlfriend) also became a partner. They were NOT in any form of relationship. I received a pop-up as well.
Then, I put my mods back in. Loaded the game up and the exact same sequence of circumstances happened. Secret Lover and Romanced Sim both became Partners.
The last relationship change is of note, since they just had a high romantic bar and I did not make them official/committed partners or secret lovers anymore. But the glitch set the relationship to "Partner" automatically.
Edit By Crinrict: Adjusted pack tag to fit conventions of the forum.