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False hope: resetting the SMC didn't change things, the Mac still freezes when left alone for some time. It may just be the longer display-off time in the Energy Saver prefs that increased the time before the freeze. Hope Apple fixes the bug quick, as affected Macs are close to unusable, especially for Time Machine backups.

Progress update: resetting the SMC seems (so far) to have solved the issue. Immediately after resetting the SMC, the Mac was incapable of setting secure connections up (no connection to https sites in Safari, no authentication in streaming apps or antivirus software, and System Prefs > Apple ID claiming new authentication was required); I do no know whether that is related. Anyway, this again was solved by signing out in Apple ID; the process was buggy, two attempts were necessary, but after doing that and restarting everything was OK: still signed in in Apple ID, https connections and authentication working, and (so far) no freeze when the Mac is left alone for several minutes. To be on the safe side, I also set the display off time to 2 hours in Energy Saver prefs for Power Adapter.

Exactly the same here, with MacBook 8,1 (Retina, 12-inch, Early 2015): when left alone for a few minutes it becomes totally unresponsive and the only way forward is to force swith off or reboot (by pressing the Power key for several seconds). When that happens, the back lid is quite hot. This was with the MacBook in closed-display mode plugged into a LG Ultrafine 4K monitor, which provides power to it. This all started when installing Public Beta 4 (19A526h).