I would wonder if there is a behaviour running where you are not expecting it to and that is involved – and dependant on the actual ‘actions’ taken on the form, it runs sometimes and not others ?
Test by running in ‘debug’ mode’. As you enter data into the form as the users would you would then see in the debug pane any/all behaviours that are executing.
You may find it useful to track if you rename your behaviours to something meaningful (the actual event action as well as each component in the actions if there are conditions in place. open the behaviour and you can rename the behaviour from the default (ie button is pressed to something that shows you in the debug what behaviour is involved
Hope it points you to the situation, but if not, please send over an export of your form (with details of the actions required to replicate this) to us at firstname.lastname@example.org and we can have a look.
PS confirm as well in the email that you are running the latest version (1.14)