Left-fixed visualized differently between preview and actual reading mode

[tsteckenborn]

Given

> [!multi-column|left-fixed]
> 
> > [!blank-container]+
> > 
> > ```button
> > name New Meeting
> > type note(undefined, split) template
> > action Meeting
> > ```
> 
> > [!blank-container]+
> > 
> > ```button
> > name New Mail
> > type note(undefined, split) template
> > action Mail
> > ```
> 
> 

it is displayed correctly within the preview:

yet seems to have malformed visualization in reading mode:

Updated from a version around 0.6. Was working fine back then.