Why am I seeing, "This question was not displayed to the respondent"?

The final survey question/block in a survey my student is currently running is usually (but not always) not being displayed to respondents. Upon reviewing our data so far, we noticed that most of our participants have not responded to this survey block. When I review individual respondents' data, I see that this question is typically greyed out & it says "this question was not displayed to the respondent." There is no skip or display logic & there are no branches that would affect this question. We did not change the version between the time this study went live & now, so it's not that respondents are seeing different versions of the survey. And it cannot be that these participants simply closed out of the survey at this point because there is a second linked survey that comes after this block, to which most people have navigated. Of over 100 responses to this survey, only 9 people have had this problematic question actually displayed to them. Who sees it & who doesn't seems random: one or two of our first respondents, one or two of our last respondents, & then a cluster in the middle of our completed respondents. We tested extensively before we started study recruitment & never had any problems with this. Our first study participant said she could see all blocks & questions in the survey (& her data bear this out). I tested the survey again myself this afternoon & I was able to navigate through all parts of the survey, including this last block. I use Qualtrics regularly for my research so I'm decently adept at using the platform at this point; I'm totally baffled & have no idea what could be causing this. Please help!