I do a similar thing:
Before, tablet mode was enabled the moment the keyboard was detached (no matter the position of the keyboard-detach and tablet mode was enabled). Now, the only way to get tablet mode is if I flip the keyboard back (fully folded backwards). Once that occurs, tablet mode appears and then I can detach the keyboard and tablet mode stays.
I am not sure if the feature was changed to the keyboard needing to be flipped back and then detaching or if it is a bug with the recent releases.