The same thing happened to me. I have bought for rent quite recently, maybe a month or two ago. At first all was well, and then one of the tenants died (he was the only one in this unit) and since then all units in this rental have a max value of around 4/5$.
I have 2 rentals, one where I live + one more unit, it's a duplex. And the second rental has two units, also a duplex, where the issue is occurring.
The rental unit on the property that I live at has no problem, it's only the second property. After the tenant died I went to look for new ones and they all had max rent of around $5, but because I had a pre-set rent for the previous tenant I didn't want to lower it so I left it as it was, leaving the unit empty. Then randomly a new tenant started occupying the flat - I did not make an agreement with him, so not sure how it happened, he just appeared, but the rent was stil set to what it was before so I didn't evict him hoping for even a slightly higher income.
When it was rent collection day, I got a notification with the amount I have received and a line saying that any owed rent will show in the business panel - nothing showed so I wonder if this is a generic message for just in case, or if the game is reading the income as if it's receiving only the 4/5$ but because the rent is pre-set to higher, it's thinking there is something outstanding. Not sure if it's connected at all or if the income thing is an actual issue, I just know that I received less than I should have for all the 3 properties combined.