The attribute marked_at is not updated when marking an already marked episode using the Premium API without explicitly specifying a value for the attribute, but it is updated when using the website or when using the Premium API if the episode was not already marked.
According to the API documentation:
Implementation Notes
Set marked_at to NULL or leave it out to use the current time.
Reproduce:
1. mark an "Unwatched" episode to "Acquired" on the site, either using the Premium API or the website => the attribute marked_at is set to the current time
2. mark the same episode as "Watched" using the Premium API without explicitly specifying the marked_at attribute or setting it to NULL => the attribute marked_at is not updated
3. mark the same episode to "Skipped" using the website => the attribute marked_at is set to the current time
I think it is a bug, because the marked_at attribute is updated when using the site but not when using the API.
EDIT:
if this is in fact a bug, it may be worth checking whether the same happens with the attribute voted_at when using PUT voted episodes or PUT voted shows
