Which is just so damned strange. There isn't any reason for it to do that just because of this plugin.
On the page itself, there is just that button - and, you *must* click it before it loads any of the who viewed data - just viewing the topic that data is not there.
So that leaves the underlying code hooks. One just says hey, if someone is viewing this topic write to a database field they are viewed this, the other is only called when you click the button to get that list of viewers.
So damned odd...
Let me play with this on my end, I doubt I need to Facebook it up as I can just look at the RSS feed directly and see what's up. If the image is not included (or is correct) in the RSS then this going to be something out of my hands - though it may be with the Zapier thing you have crafted.
Going out for Greek food in a bit but I'll give this a look over when I get back. If you can, could you PM me the specific Zapier app (or custom code you are using there), and I may later need ACP access to debug this correctly (if this is solvable on my end)
(Sorry, this is AA, doing other account stuff today)