pdfs not saving as pdfs after using save as function

I use the stamp and digital signature to verify pdf documents.  After this, Adobe asks me to save the changes.  I have always used "save as" and then saved the document to indicate its been verified.  However, today, I cannot save or save as any documents that i have signed.  Adobe takes me through the process but what i have in the folder is a document unrecognisable in any format with half the name removed.

Example

original pdf :  Laptop_PMNT  (openable, readable and editable by adobe) The adobe pdf file symbol indicates that it is a pdf.

Post stamping and digital signature,Saved as:  Laptop_PMNT_ss

Resulting file saved on microsoft 365 :  Lapto (not reopenable, letters chopped off the original name) Empty rectangle indicating it is an unknown file type.  Asks me to search the microsoft store to find a suitable programme to open the file.  Microsoft has no suggestions.