I’ve had a response on my support ticket that the issue was found and they implemented a fix on a test site of mine. I confirmed the fix is making things work as expected. They said to expect an official release soon to fix the issue.
I am also experiencing the second issue on multiple sites. Reverting to 5.7.10 fixes it immediately. I’ve sent in a support ticket just now. Specifically when I have a repeater inside Flexible Content, the functions have_rows() and get_sub_field() return NULL for the repeater. The data IS accessible if I get the FC field with get_field then move through the array. But that takes major rewriting to fix the PHP template files.
