I was exploring and discovered a ruined portal under the ocean that didn't have a chest. To my knowledge, this is not supposed to happen any more as the issue was resolved in MC-182652.

Seed: 5708653125691832534

Coordinates: -1066, 40, -1152. The ruined portal is in the -Z direction.

It appears to be a underwater variant of the ruined_portal/portal_7.

I found it using optifine on a world originally created in 1.16.5, but this was a new chunk in 1.17/1.17.1. I successfully recreated the bug in vanilla 1.17.1.