HIPAA and BAAs are for lawyers to argue about after your medical records has been exposed, and if attempts to remedy the situation deteriorate.
If this encryption genuinely allows structural data retrieval - eg common invoice templates or medical reports - then this is absolutely a HIPAA problem, and once a facility is made aware of it they would need to respond.
FWIW, this message encryption is also accepted by UK NHS, in fact it's virtually the standard.
The mitigating factor is that typically the user is fully authenticated before being allowed to access the encrypted data...
AS OP mentioned, this is not about assumptions but laws and their interpretation during consideration in a court of law.
Sounds spammy, but this is industry norm from personal experience on Ironport and Exchange. Downside is the spam imitations of this that are likely quite successful at phishing creds from corporate users.
Should clinicians just ignore this or should they demand a response?