Outlook for Mac (16.57) attachments not being sent

Hi,

Recently, several different clients are telling me I've forgotten to add attachments - although I haven't! I'm attaching files (usually a PDF) that are way under the limit and hitting 'send'. Then I'm seeing that email in my sent items sometimes without the attachment – it's like it's been removed between hitting 'send' and actually leaving Outlook. Any thoughts on this? It's getting embarrassing and very annoying now! I'm running Mac OS Monterey v12 beta.

Thanks in advance!