The 12.84GB matches the encrypted partition on a 16GB model. And it seems that you have the proper password to unlock the partition. That’s good!
Online/offline in this case means whether the drive is used or not. A PC may contain drives that are not activated and sleep. This is fine.
Maybe the encrypted partition was removed while the filesystem was still writing files. That way it can become corrupted and needs a filesystem check.
Please Note: As this alters the filesystem, you have to be careful and it could potentially also destroy data on the stick or make recovery more difficult. Depending on which errors need to be fixed on a filesystem, the outcome may differ. In some rare cases, a filesystem may stay corrupt and dataloss could occur.
If you have no backup of the data - and having valuable data absolutely demands for creating backups of that data - you should ask someone experienced with PCs and troubleshooting. E.g. an IT guy could create a dump of the filesystem before trying anything to fix the filesystem. That way you do not alter the data and have better recovery options.
That said, having a corrupt Fat32 filesystem is also a common thing and usually just using the filesystem check that can be run from Disk Management by selecting
partition->right mouse button context menu->properties->Tools->Error checking->Check should fix it.
A corrupt filesystem may be one reason, why it does not get automatically mounted on Windows.
Another reason could be that the encryption keys got reset before in order to recover from a lost password. If that was the case, the filesystem is in a blank state and before using the Nitrokey storage, a disk format is required. When the encryption key has been rolled, the data is lost on the device anyhow and cannot be recovered.
It could also be a bug. While my Nitrokey Storage works just fine on current Windows box, there have been some issues reported that look similar. I would contact firstname.lastname@example.org, then.