The NaN issue using dynamic commands started in November with an update. "NaN" (from dynamic command) spreading through my vault · Issue #910 · SilentVoid13/Templater · GitHub
I use dataview for modification date now:
updated: `=dateformat(this.file.mtime, "DDDD, HH:mm")`
which works in both Live preview and Reading modes, but it can’t be in the frontmatter.