Please help us help you and fill where relevant:
Your LimeSurvey version: 6.16.4
Own server or LimeSurvey hosting: we host ourselves
Survey theme/template: we started with Vanilla and customized it
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Hi, we have a live survey that has been successfully running since April 2025. Between April and now (Feb 2026) all has appeared to be running well and we have kept up with updates to LimeSurvey along the way. We aren't sure what versions we had between April and Feb, but as of right now we are using v6.16.4. 

Today we looked back on the respondent data and found that a small window in mid August (approx 5 days) somehow permitted users to submit the survey but the token fields are blank in the responses data export. The survey is closed access and requires a token for the user to gain access. We send email invites and the users key in their tokens or use the URL with the concatenated token value in the string. 

Are there any known reasons why the token values would be missing in the responses data? We even have all of their contact information with the submission - but its just the token values that are missing from approx 25 entries. After this small window of time, the token fields are back to normal with values. Before this window of time all of the entries have token values also. It was just during these 5 days in mid August. 

Did we possibly have a version of LS that had a bug related to this challenge? Could it be something else? 
Any help is greatly appreciated.
Thank you