Strange soft spot on duck egg

[Tapioca]

Has anyone seen this before? The egg is hard shelled except for a small spot. It looks like tiny calcium deposits on the soft spot. I've been getting one egg like this a day for a while. My ducks are fed layer feed with free choice oyster shell.

 

[Miss Lydia]

Has anyone seen this before? The egg is hard shelled except for a small spot. It looks like tiny calcium deposits on the soft spot. I've been getting one egg like this a day for a while. My ducks are fed layer feed with free choice oyster shell.

Yes I have seen things like this. This is info on chicken eggs but I think it can pertain to eggs in general.

https://www.backyardchickens.com/a/common-egg-quality-problems