I could not find any other thread with my issue but found this to be the closest.
For me, I started using new app (STSC) around November last year, and slowly migrating my stuff, like automations, scenes etc. to the new app. Then I found out, Google Assistant first appeared with duplicated scenes, like one created in classic app, and new one in the new app:
[SCENES] Good Night
And then 2 days later, I found in Google Home app that old scenes created by classic app disappeared, only with ones in the new app.
This was where my problem started. For me, using Android, I could use and control with the scenes without any problems, although I had to reassign them to my Google Assistant routines. But for my wife’s iPhone, scenes were not appearing in her Google Home app. Basically she’s not able to assign scenes to her routines because the “Adjust scenes” option in her routines were greyed out.
Is this an issue created by the new app or is it created by the last update? I third to disable and reenable scenes available to Google Assistant in SmartThings app (both classic and new), sync devices on Google side, tired creating new classic app routine, all to no avail.