Search exclude doesn't seem to work in some cases

On the pop-up help for general text search there is the text:

Use a dash to exclude the coins matching with a word or expression.

Example:  Martin Luther -King

That example works, but many other uses have been failing for me.

For example:

• Search for Coins with Gregorian data 2025: 237 results found.
• Search for 2025 and “NumistaBug”: No result has been found. (As expected.)
• Search for 2025 and not NumistaBug ("-NumistaBug"). No result has been found. (?????)

What am I missing about excluding items from search results?

It appears that you need to specify a positive term in the general search results before a negative term works. 

Searching for Gregorian date 2025 and “2025 -NumistaBug”: 236 results found.

(One less than expected. It appears one coin ( N#178135  ) is not fully indexed.)

The search tips lack a full description (and lack a link to a full description).