Exchange Online Encrypted Message - message.html vs message_v2.rpmsg attachment

Hello all,

Can anyone help point me in a direction?  Our HR department sends an encrypted file to one of our benefits providers weekly.  Last week, our provider could not open the message, and the same happened this week.  We noticed that the message was previously sent with a message.html attachment that would send them to an M365 login/OTP to open the message.  For the last two weeks, the attachment is message_v2.rpmsg.  Our vendor is unable to open this message.

We are using Outlook C2R clients on our desktops.  I'm unsure of the vendor's mail provider, but a quick MXToolBox lookup doesn't immediately present itself as M365.  Did something change recently with message encryption?  We are using OME, and some Google searching seemed to point to encryption being recommended via Purview now.

Any insight is greatly appreciated.

Thanks

Steve