Hi everyone! We're aware of the issue (and very sorry about it!) on the Gear S2 where the watch face freezes when the screen turns on while dim mode is not enabled; we are rolling out a fix as soon as possible.
For now as a workaround, we can confirm that enabling 'Watch Always On' mode will fix the issue.
We will update the community as soon as the fix is available.
Thank you for your patience!