H.265 sky distortion (banding), only in h.265

I often shoot underwater, and there is no problem with the preview of the film. When I export using h.265, I find that there are many bands. So I did a test, as shown in the figure. When my material is previewed in PR, there is no problem even if it is enlarged to the full screen, as shown in Figure 2, but when I export it with h.265, the strip appears, as shown in Figure 3. No matter how I change the bit rate and other parameters, the stripe always exists. I began to suspect that it was caused by color depth, so I used afterencodes to export 10 bit video, and the strip still exists, as shown in Figure 4 (played with potplay).

But when I use H.264, all the problems disappear. Even if it is 8bit, there is no stripe in any case. As shown in Figure 5

This shows that there is a problem with PR coding. How to solve it?