Starting sometime last year and with increasing frequency, students' submitted papers will randomly lose formatting elements in the SpeedGrader view. It happens with PDFs, docX, Googledocs, etc. It seems especially to appear if the file originated in a browser-based version of Word 365 or in GoogleDocs, but it does happen otherwise.
Indents often disappear. Additional spacing may appear/disappear. Lines may shift onto a new page, causing a student to appear over a page limit....
It's maddening. All of the recommendations I've seen suggest to use a PDF, but that doesn't always help (indeed, a student just emailed about losing indentations with a PDF). I have also noticed that Canvas's preview changes Word's new default Aptos font to something else, but that's not the only change.
In practice, I have students make a comment about the issue, and I check the raw file they uploaded, but I'm hoping for a fix that stops the problem.