Sorry, I should have made it clear from the beginning.
I had already addressed this question before here in the forum when I edited a picture I took with this old camera I bought a month ago.
Today, while editing another picture, the bands showed up again but, this time, I researched the Internet to see what they mean, and found this post where they were doing the steps above to reveal the noise pattern.
I’ve asked for this test to confirm what they say on the web, which is, basically, the sensor end-of-life, and that a newer camera wouldn’t show that pattern.
So, although I still get good images when the light is good and uniform, it seems that the camera shows its limits in recovering deeper shadows.
Here’s the picture I was editing today (exposure + filmic):
EDIT: I tried using that black frame in Rawtherapee’s raw tab to reduce the effect, but no success.