While a definitive answer can only come from the origin of the message and the restrictions, there are some points to make.
The general idea of restricting passwords in such a way (especially with the tmp-part) suggests something is not as it is supposed to be and hinting at plain text passwords. Yet for the benefit of the doubt, they might use a special hashing scheme that hashes the first three characters of the password separately, allowing them to check for temporary assigned passwords.
This might pose a threat, depending on what comes after the
tmp part: if thats just 5 numbers, the search space for enumeration on such passwords is tiny, for example.
In any case, this restriction hints at them using passwords to store information that does not belong there (but in a separate field in the database) - and while it doesn't have to pose a threat by itself, deploying such techniques is generally a sign for bad software engineering.
This by itself can pose a threat: Bad software developers are more likely to make mistakes on many levels that only wait to be found and exploited.
Whatever might be the reason for them to deploy this restriction, it is a sign of bad programming and/or bad software or database engineering and thus a sign of danger; you should use a disposable, strong password and basically consider the account compromised to be on the safe side.