Weird! I have a few ideas, but no obvious solution...
First, let's look at the popup... Try to change the Navigation type to the "normal" window, not a popup. This way it will open a new page and will pass all parameters to this mashup via HTTP GET query parameters. In other words, you should see the content of that infotable in the URL. Please check it and tell us if it's there or not.
Second, what if you create a new pair of "minimal" mashups, and only put that logic in them? In other words, can you try to have a minimal reproducible example? If it works (hopefully!) then you can check the difference between your "real" mashup and that. If it doesn't -- you can submit it to PTC Support.
Third, assuming it's a problem with mashup parameter TYPE, i.e. the fact that your MashupParameters is an INFOTABLE. What if you convert it to JSON, and pass as a JSON, or a JSON.stringify() even? Then, on the receiving side you'd have a few expressions, which "convert" it back to individual values (one Expression per column). To convert INFOTABLE to JSON you can try this trivial extension, I hope it still works with the latest ThingWorx: https://github.com/vilia-fr/twx-infotable-to-json-ext
Fourth, assuming nothing else works. You can bind your clicked or selected row to a Session parameter, and access it from your popup. Gotta be careful, as this will create side effects, will make your code harder to read and maintain, and will also trigger a REST call to save this dummy session parameter on the server side, too.
Update (just thought about it) -- before doing any of that, try to bind individual attributes from Selected Row(s) to individual mashup parameters, so that you don't need to deal with INFOTABLE parameter:
/ Constantine
