With all that description, I start to wonder if maybe there is a problem with that specific wip file you're using. We certainly have seen that before, both personally and helping other people. Are you able to share it? Was it converted from some other format, at some point? Did you make the whole thing yourself?
Please do something like this. Maybe in a new layer, or in a corner somewhere, create a new marker which does not look anything like the others you've made (just so we don't confuse it with your others). Save the file as SVG and share it with us.
I don't think the extra monitor would have anything to do with this. Or I should say at least I haven't seen any reports about it.
Sharing the file with us would be the best way to investigate further. But if you're not able to share that file, here are some more questions.
How do you make the marker? Have you noticed that a marker made with one kind of object behaves like this, but made with a different kind of object it doesn't? I'm wondering if possibly using an LPE for example, and converting that to a marker, or....I actually don't know if this is possible, but converting a pattern to a marker, might cause some strange behavior? Or maybe using a gradient mesh and converting that to a marker, might do something strange?
Edit
I just had a stray though! After you make a new marker, open the XML Editor. On the left side, click on the plus sign beside the item near the top, which has "defs4" in it. Look at the bottom of that expanded item, and there should be the new marker there. (The newest marker goes to the bottom of the list there.) I wonder if clicking on that new marker in the XML might do something?
When I try it, it doesn't do anything. Normally when you click on something in the XML Editor, it selects it on the canvas. But because the new marker disappears after you make it, there's nothing to select. But I would at least try it.
Or maybe you can determine whether the marker was actually created or not. Make another new one, and look in the XML Editor, to see if there's a new one there? If you don't see it in the XML, that means it's not being created. So at least we can find out if the marker is being created.