Data disappears from encrypted box once it is opened without cypher

I did a test project with only Dart (no platforms) and this happened:

I opened an encrypted box and stored some values, then next time I opened the box without encryption and when I check the box.values they are empty.

Is this expected in any way? How can I check if my box is encrypted?
I was expecting that I could check whether the db is encrypted by getting some garbage data.
Even if I open the box with encryption again, it is always empty after first time.

The issue names no source file or test. Reproduce it in the described Dart-only project by opening an encrypted box, storing values, reopening it without a cipher, and then reopening it with one; determine the expected handling and document or test the resulting behavior, including how encryption status should be checked.

Written by the indexing model from the issue text.