Strange bruises near toddlers spine

I saw a similar post a while back but wanted fresh opinions. My 18 month old gets small round light brownish bruises on his back near his spine, look like when a banana goes bad. He’s had these come and go for over a year. They last a couple days to a week and then disappear. There’s usually only one or 2 spots at a time. Mostly lower to middle always near the spine. His blood work came back normal, and no other symptoms besides eczema in other spots of his body. He’s been to several doctors who said it’s nothing to worry about, but nobody can’t exactly figure out why. And none of them said they see this frequently. The obvious culprits would be high chair or 360 car seat. But i check the back of both and they are cushioned well. He’s active and falls a lot but nothing that would cause trauma to the back. Anyone experience this and be able to pin point? It’s annoying and obviously avoidable but we can’t figure out. Appreciate and feedback from experience (not sure how to post a pic)