Is this a B2B or B2C survey? Once thing to note is that as anti-spam filters become more and more sophisticated, they are testing the links within the emails. What this means for inline questions is that you will get a significant number of invalid responses since the links in your email are recording actual answers even though the customer may not have clicked into it. I had to stop using inline questions because of this.
If the above is not the cause of the issue, then other things to consider is what the next question(s) in the survey are - do respondents deem the next question(s) relevant?

This survey is B2B; however our organization also conducts B2C surveys. I had not considered that that spam filter could actually click on links.
Do you know if there is a definitive way to determine if the spam filter provided the response instead of the intended respondent? ... or is this based on your best guess based on which questions were answered. I ask because in the case of my most recent survey, the NPS question is Q1 and the follow up open-end is Q2.

I'm not aware of a way to tell the difference. The only way we found out is through the closed-loop process when we followed up with customers who provided us negative "responses" and they said they never even clicked into the survey or opened the email. Further investigation with our security team uncovered how spam filters work.

Thank you for this information. Our organization is still assessing the effectiveness of using inline questions.