Came here to report something very similar. On iOS when pressing anywhere > select > then dragging the selection handles upward through a lot of text, as the screen is moving up the selection handles will just stop and disappear. It only happens when dragging the selection upward. Dragging downward seems to work fine on any length of text. I can also replicate the select all issue too.
No I’m using the default dark theme, but I have a few snippets I added to change the background color and some text and heading colors. No themes or plugins installed though. I tried it again today and it only seems to be happening once I reach the very top of the page when selecting upward. Previously it was doing it at any random spot. I tried turning off the snippets and it still happens.
I’m now also noticing some page stuttering/pausing as I’m scrolling up and down a long page.
After testing a handful of pages, I think it mainly fails on longer pages.
I hadn’t noticed the insertion-point jump until you mentioned it. For me in the middle of a long page it disappears, and I might find it at the bottom, at the top, or not at all. Starting with it near the top of a page, I saw it jump to the top.