I migrated to the new ST app, and all my routines and scenes disappeared, after I created a NEW routine in the new app. All the old routines/scenes seem to have disappeared into thin air! This happened once before, but when I quit and restarted the app, it seemed to fix itself and they all came back. Not this time - the difference is that I happened to create two new scenes in the new app this time - and those were the only scenes that showed up!!! Help!!
I suggest try uninstalling the new ST app, reboot your device and perform a fresh install of the app. Sign in and see if your automations/scenes appear. Then try to create a test scene to see if the problem reoccurs. I do not remember seeing this particular issue being reported by others.
Now time for my lesson of the day. You mentioned routines disappeared in the new app. Routines were a feature of the Classic app only. In the new app, routines do not exist but automations are their replacement. 
I have same, all but one scene dissapeared after creating room. Disgusting app, as if made by kids, why break something that worked
This happened to me this evening. Not only are both scenes and automations gone but scenes are also gone in the IDE (can’t fetch scenes). Smartthings didn’t answer the phone and complaint/report form keeps giving me an error. How can I rely on this to manage my home, especially when I’m away, when weird unexplainable things keep happening?
It has been weeks since this happened. I have uninstalled and reinstalled the app multiple times with no benefit. IDE still says it can’t fetch scenes. All automations continue perform, I just can’t see or change them.
I cannot create a scene or automation using any devices. The device options in create scenes or automations are disabled. I can create a scene that changes the mode only.
All other users can see scenes and automations as normal.
I contacted support on January 23rd and after an initial flurry of uninstall and reinstall, reboot and other standard procedures (none of which helped), the rep who was helping me has simply dropped off the face of the earth for well over a week.I have emailed several times with absolutely no response.
I have three locations running Smartthings. Does anyone here have any clue? Because of the "can’t fetch scenes’error in the IDE, it would appear this is a cloud problem and not an app problem.
As an update, after a few more calls and email exchanges I finally got to tier two support. Eventually this was a problem with how older Smartthings accounts were converted to Samsung accounts. It was affecting a number of users and required some additional coding work.
Tier two support was incredibly helpful.