RenderForwardOpaque.Render -> Clear takes too much time with a very simple scene.

I have a problem with RenderForwardOpaque.Render -> Clear. Although it’s only rendering a scene with 3 draw calls (it’s a simple UI with a background and less than 5 simple UI images). My device is iPad Mini 4.
Most of the time my game is running stable but this issue occurs randomly (maybe it started when the banner ad is loaded/refreshed). And from that time, the profiler shows spikes of RenderForwardOpaque.Render -> Clear. Fps dropped down below 60fps and very lagging.

I also see that, if I try to interrupt the device, it might help to recover to normal state (try several times).
My Unity version is 2018.4.12f1.
Could you give me any advices on this please?