Welcome to the community! Kindly please be informed that this is a bug already identified and fixed (which should be released shortly in both the KoBoToolbox servers). You generally get this type of an issue when you design a form with matrix question, save the form and then try to edit the form back in the form builder user interface.
Note: Please do not try to edit your survey project if you have a matrix question in it. If you need to edit the same try editing your survey form in xlsform and redeploy the same. Doing the same should solve your issue at the moment.
Maybe you could have a look at the workaround that has been discussed previously which shows a simple way of editing a matrix question in the xlsform:
Have a great day!