there are two ways that I know where workbooks can lose db credentials.  Are either of these the case?

- When workbooks are published, the user publishing them should always remember to embed credentials instead of requiring a prompt

- On server, if ownership is transferred to another user, then credentials will be lost.  Go vote this idea up:  https://community.tableau.com/ideas/6698

There's one other way embedded credentials can be lost:  Tableau Server software update.

It appears random, doesn't happen to the same workbooks, but I and some of my users have experienced workbooks switching from Embedded to Prompt after a Server version update.  I've also had some people lose their Publisher role after an update.  Both instances of it happening are very small (maybe 0-4 objects after a version update).

That may have been the result of a defect that has been fixed. There were several issues involving embedding that was resolved in 9.3.1 - 9.3.1 | Tableau Software