Sims friendship randomly maxes out?

There is another way. Seeing that threads are closed just because it was said to be the solution (but it didn't work for me however) i found your similar question where i could share my experience of solving it now. And that is without removing any mod packs or adding patches or clearing caches. None of that sort. As long as you're able to access the cheat menus in the game, you could do it, possibly. This is after I got the game updated yesterday 6th Nov 2023 from EA.  
I see that most have not been able to use the code:
modifyrelationship 1stname1 lastname1 1stname2 lastname2 -100/100 friendship_main
cheat code ever since the game got updated. I tried finding for a solution to change a bad friendship level to better but i could not as with friendship_main. The error was on the value string. However with romance_main , that only worked. And that even worked without using the last names of both sims.
The workaround i've managed to unfold isn't mentioned anywhere. So here it is:
First shift+click on the involved sims/the played sim. Then click on "More Choices"
Find and click on Cheat Relationship. You then will have the options to Add or Remove Relationship Bit.
(Before that, you'll have to know how the sims feel for each other to choose which to add or remove from the bit. You can find that out from the relations tab [or press 'R'] which is below the screen on the right and then moving your mouse over to the sim involved with. The info will be shown on a small icon on the left of 2 top and down bars of that sim.)
Knowing that, will help you select from the list in the Relationship Bit. If you have the best friends type of option to choose from the list to add, that will be what you should select and apply upon. You could also remove those that is listed to cause bad relationship with the involved sim. Applying the friendlier option/options to both sims will change their bars to green immediately.
You may then apply the romance_main cheat code if you want the involved sim to be freshly be in love with your played sim.
In your case of getting maxed out and then fluctuating again to bad interactions, I'd suggest to try this out and see.