I was able to recreate this in a fresh install of WordPress using TwentyTwentyFive theme and modifying the code enough to create two custom ACF Gutenberg Blocks using acf_register_block.
In one of these blocks I apply a WYSIWYG field that is initiated on page load (initiation is not deferred to onclick). The other block contains a single text field.
I then begin adding these blocks to my page. WYSIWYG block first, about a half dozen text blocks, and then I end with another WYSIWYG block. If I then go back up-page and insert a block of ANY type, as soon as I insert the block my cursor focus is hijacked and placed in the last instance of a WYSIWYG field.
This is occurring on at least two of my production sites that are both using ACF Pro 6.3.12. And again I was able to replicate this with a fresh install of WordPress also using ACF Pro 6.3.12.
Is anyone else experiencing this issue?