Time Machine - external drives not showing in TM Backup when unplugged

Hi everyone, i found a scary behavior of my TM Backup.

I am backing up an iMac internal drive and an external TB-SSD (Pictures) onto an external USB Harddrive using TimeMachine.

When the external TB-SSD is mounted while i am accessing the TM Backup everything looks fine and i can restore my pictures.

As soon as the external SSD is unplugged, the backup of the SSD Data (my Pictures) does not show up in the TM Backup.

This makes my fear a bit. So whenever my external Ssd fails or is lost (= is unmounted), i can not acces my pictures from the TM Backup?

I am running 10.15.5 on an 2017 iMac 21’

Any suggestions what is going wrong here?

Kind regards