Capulettium Plus does not prevent movement.

[Mordent]

Doesn't look like a bug to me. From the wiki:

Instant and ongoing vocal paralysis until chemical fully depletes, also prevents the use of all emotes. Makes the victim appear dead if they're laying down or sleeping.

This essentially lets you play possum. You're unable to speak (excluding telepathy or machine talk), but you can make yourself appear dead in a pinch. Know someone is hunting you? Inject yourself with this just before they find you, lay down and they'll mistake you for a corpse! You retain full consciousness and control of your body, so you can back up and ambush them when they least suspect it.