Is it really rare for l5-s1 bulging disc / minor herniation to slip back to its place compared to l4-l5?

First, discs don't : "Slip" back into place but the majority resolve just fine over time 90% without any surgery. If one were to do a follow up MRI from the first one over a 6 month period, 60+% chance of noting the disc resorted or shrink in size . Also, disc bulges are normal findings on imaging studies of the spine and even significant disc herniations can get better without surgery.