Smartstream - unable to export multiple spread elements

[FishTetris]

I frequently export jobs with multiple spread elements, and this is the first time I've encountered this problem. I'm using Smartstream with InDesign 2019.

Today I set up a typical 13x19 imposition, with the following spread elements - a 13x19 PDF containing diecutting eyemarks, a small .indd sheet counter element, and another 13x19 PDF with additional special treatment eyemarks.

For some reason, on export, only one spread element was appearing in the final PDF. Changing the order of the spread elements in Smartstream would allow a different spread element to export, but still only one element. My troubleshooting attempts so far:

1.) Confirmed that the PDF spread element backgrounds were transparent (in case the spread elements were physically obscuring each other).

2.) Confirmed that all spread elements were linked from the same source folder.

3.) Re-exported an older job with multiple spread elements, which worked as normal.

4.) Created a new document and a new imposition with multiple spread elements, which again resulted in only one spread element appearing. (Tried this multiple times using newly created spread elements and spread elements from past jobs with the same result).

5.) Restarted my computer, repeated steps 1-4.

Maybe I accidentally changed a default setting somehow, but I can't see anything obviously different in my imposition settings from previous jobs. Any help is appreciated!