Ideas
I tested this with my child. I had created an imaginary friend, and they were best friends with a strong relationship. Everything was working normally.
After making my Sim go to sleep overnight, the imaginary friend disappeared from the game. The next day, when I tried to call or summon them again, it was as if I was creating them for the first time.
There was no more relationship bar between my Sim and the imaginary friend.
The friend no longer appeared as an imaginary friend or toy, but as a regular Sim in both appearance and behavior.
The relationship had completely reset to zero.
In other words, the friendship no longer existed. The imaginary friend was reset and lost all of their imaginary friend characteristics. And now, whenever I transform any imaginary friend, it takes on the appearance of a regular child Sim.