Loose End Cap?

Hello!

I had a mechanical a couple weeks ago where my derailleur hanger broke. I had to take off the rear wheel to get bike in an Uber.

After attaching a new hanger today, I noticed my cassette seemed more loose than what I recall.

I’ve noticed my rear hub end cap comes off without added force if I tilt the wheel facing downward.

Question:

Looking inside the end cap area, am I missing a piece to help it fit snug without falling out so easily and allowing my cassette to be snug without play as it was before?

Is this piece the “DT Swiss Cassette Seal”?

My rear hub is the Shimano fh tc500 hl.

Thanks for any help!