I had a rought time figuring it out but finnaly I found the bug,
on line 1390 of forms_model.php
"WHERE lead_id=%d AND field_number = %d ", $lead_id, $field_number - 0.001, $field_number + 0.001);"
That fails when the wordpress Locale is something like French, the commas get interpreted differently somewhere in the prepare statement and the query fails and you get stuck with the short version of the string.
changing the line to this:
"WHERE lead_id=%d AND field_number = %d ", $lead_id, $field_number);"
will fix the issue.
I saw other places in the code where Between is used with the same logic of using floats instead of Int in the SQL query. I'm not entirely sure why it was done, I'm sure there was a logical reason at some point in time (or debugging and never got put back).
Either way, I hope it can be of use to others.