Referring to this thread in LS forum:
https://www.limesurvey.org/de/foren/design-issues/113928-reopening-a-completed-survey-erases-existing-responses-is-that-normal

We use an old 3.5.0, but others have the same problem with 3.15.9.

Issue:

• we enabled "token-based response persistence"
• we allowed "multiple responses or update responses with one token"

We get occasional reports from respondents and our quality control team, that random variables are cleared upon re-accessing formely submitted surveys.
In our case it's never the complete survey that is wiped, but from what I have seen so far it seems to be a complete question page in each instance.
Order of magnitude: maybe 4-5 % of re-accessed links.

I can rule out that it's the some kind of user's fault, as there were 2 instances where the questions that got cleared upon re-opening the link
1) were mandatory List (radio) question where the users can't "de-select" once an option is selected
2) is itself always shown (relevance=1) and was on a page with relevance=1 (so no messing with earlier questions that could have made the page / question irrelevant)