Lines shapes aren't appearing in Preview

Hello,

I wonder if anyone has come across this?

If I draw a line shape on the screen (with or without an arrow head), it doesn't appear when I preview the screen. It's fine in design mode, however.

I've worked out that although the shape outline may say 2 pixels in the Format menu at the top, if I right click the arrow  and choose Size and Position, the Height is zero px. If I change this to 2 pixels, for example, the arrow now DOES appear in Preview, but it always skews itself and isn't true horizontal or vertical. If I then use my mouse to straighten it up back in design mode, the Height changes back to zero pixels and the arrow disappears again when previewed!!

I'm on version 3.17.16117.0 - so not the latest, but not terribly ancient either.

All very curious! Brand new laptop, too, running Windows 10.

Best wishes,

Martin