Don’t know what is causing this, but I have a file where zooming extents in Layout space pushes the actual extents over to the left, off of screen-center. It seems to be the linked file title block I am using.
When trying to de-bug, I completely deleted all the title blocks, purged the link from Block Manager and tried again. OK, zoom extents properly centered the Layout on my computer screen. Re-inserting the block, then trying zoom extents again pushed the extents back off of screen-center.
Yet on a brand new file, I made a one-detail Layout (toolbar click), and pasted the linked title block in. Zoom extents worked fine in this file.
Files are attached if someone can figure out what goofiness is going on.