Incomplete or empty forms responses (Web App)

Hello,

I created a dozen of forms using only the web app.

I used theses forms everyday for the last 3 weeks and it worked great, with a flow getting the responses into an Excel worksheet.

But for the last three days, i'm getting trouble with the forms responses. Some of them are empty (sometime it's just a section, or it can be the whole response).

I have done videocalls with my users to see if they were filling the forms correctly, and it was ok.

I have changed the questions answers to "Mandatory", but when the user fills it, i still get empty responses.

Because I created them in the web application, and not Excel, I don't have the "Sync responses" button.

Form exemple:

https://forms.office.com/Pages/ResponsePage.aspx?id=CDbAgGRfu0CccJOUq-YBHFdk8y0tGipOsTYEQAs6qb9UOUk2VUhUWUxER1BNVFNIUTVTVVQ1RVNOUi4u

Empty answer i got after the user completed it:

The Excel available in the WebApp shows a new entry, with the timestamp, and the user, but zero answers.  

Does anyone have ever met this problem before?

And why did it worked well for three weeks, and went gradualy bananas? (it started with one form getting incomplete responses, and now 7 forms out of 15 are in this situation).

More informations:

I have tried to delete all the responses i got so far in the web app (i don't need them here, because i flow them to an Excel), but it didn't worked.

Each form is sent to a unique user, every day. It happened somedays that a user wrote wrong answers, so I went into webapp and my Excel, deleted it, and asked the user to fill another form.

Thank you for your help,