Ok, I think you narrowed it down then.
Selecting across blocks works only if the cursor is at the beginning of the block. Also, it depends on how the cursor got there! It seems it the selection doesn’t work when placing it with mouse/pointer, but it does when moving the cursor in using keyboard (coming from a different block).
Anyway, I guess the original bug is solved. Should we create a new bug report for the inconsistent behaviour selecting across blocks?
PS: not directly related, but I also have this FR on selection/blocks: