Since Apple released Pages v5 in Oct. 2013, there have been intermittent reports of this save issue. When you inititally saved that translated Word content as a .pages document, you did not mention exactly where you saved it.
I sent myself a 197 page .docx from one email account to another. On my 4GB MacBook Air, I selected the mail attachment and opened it in Pages v7.3 on macOS 10.14.3. I clicked save, and got the expected dialog, and chose to Cancel. When I selected Save again, Pages didn't hesitate to freeze. It essentially froze the entire Mac as Force Quit was inoperable as well as the Dock. I held the power key down and got the Shutdown panel, and even that could not be selected. This is the first time I have had this issue with Mojave, or Pages on this Mac. I also do not normally open Mail attachments in another application, but rather save them to disk first, before proceeding.
I repeated the steps in the second paragraph after a Safe Boot, and normal reboot. Both a save, and an unnecessary, second manual save after an edit worked fine without drama. In Pages, after the first save operation, auto-save is enabled for additional changes — unless measures are taken in System Preferences : General to suppress auto-save functionality. All this time, the Mail application was still open.
So, I am unable to repeat your second save issue here. Based on my own experiences, I cannot recommend the workflow of opening Mail attachments in Pages without first saving those attachments to local storage.