I have a very similar setup as you and have also encountered this issue, though it is frustratingly intermittent for me. When it does happen though, I have to turn the monitor off and back on, which always restores the correct resolution, although any windows on the desktop will have been shuffled around to reflect previously smaller screen.
A change that I made recently that seems to have solved the issue completely is to disable my computer's sleep settings for the monitor. Instead, I use the monitor's "Automatic Standby" feature (settings button > General > Automatic Standby) so that it powers off automatically or I simply power it off myself. This is a obviously a workaround...whatever bug that is causing this issue obviously still exists.
Feel free to let me know your experience with this approach and/or if you discover any fixes that work better for you.
