I was trying to replicate this issue in other saves and while they're not exactly the same issues and results, I'm wondering if they are connected. However, in my original bugged save I cannot further explore and troubleshoot due to coexisting and presumably connected bugs, which I mention at the end of this post.
I found an earlier version of the save originally having this issue back from April, which was seemingly unaffected by this bug. However, when I played as a household in one of the units, every single unit on that lot reverted to the EA default rent based on unit size. I then had a landlord sim buy a different, uninhabited rental lot and immediately upon doing so, the rent reverted once again to the EA default. I could fix this and it would stay fixed until I moved a sim in from manage worlds and went into their household. When I checked the tenant agreement from the tenant's side, the rent went back to the EA default price.
I also tested these same steps in a completely new, post-lovestruck patch save. In this save, I had a sim buy the lot, change all the rents to custom values and moved sims in from manage worlds. In this save, the rent did not revert until after I had the landlord sell the lot. Once the lot was sold, the rent went back to the EA default value.
Based on the new save results, I thought maybe the original issue was connected somehow to not having an in-world sim own the rental lot. In the original save I built lots, had a sim buy them, set rent prices and rules, and then sell it just to set things up, as this is a base save I am working on. But then when going through all the units in my save, I found one property made up of two units. It had one unit filled by tenants and the other was the property owner. The unit occupied by the tenants had still reverted to 0, despite having a property owner before the patch came out.
I am also having these bugs, reports linked below, which I believe to be interconnected. They're also inhibiting my ability to troubleshoot and explore this specific issue in the originally bugged save. I cannot see owned Residential Rentals in the originally bugged save from the landlord's POV, so I can not try to fix the rent prices to see if they revert on their own in that save also. Additionally, the inability to see the tenant agreement page means I cannot check to see if the rent is the same from the tenant side or at what point it reverts.
https://answers.ea.com/t5/Bug-Reports/Tenant-Agreement-Page-Broken/m-p/13903027#M283878
https://answers.ea.com/t5/Bug-Reports/Business-Panel-Doesn-t-Show-Owned-Residential-Rental/m-p/13903265#M283949