@Nadaillac thanks for reporting :-) Can you add a little more info on this issue if you expect anyone but yourself working on it? At the moment I would not know what the problem is?
On WPmaster (5.60), i have created an activity type and 3 custom fields (2 numerical - number type-, field1 and field2 and 1 alpha). I have created a form with an activity tab (I set contacts source and with to an existing contact, set the activity type, no individual) and the fields "subject" + the 3 custom fields.
Within Formbuilder, I set one custom numerical field -field1- to a number presentation and one custom numerical field -field2- to text presentation.
If I input a letter in field1 and I submit -> I get an error message, if I input "1.5" I get also a message (has to be an integer, although the field1 has a numerical type number).
If i input a letter in the numerical field2 (and fill in other fields correctly) and I submit, the activity is created, there is no error message, custom fields are left empty. There should be an error message and no activity creation.
I hope it is clearer. Tell me if you need sponsoring.
The validation on the client and also server side is based on the type selected in the Form Builder UI. This is the reason why it allowed submission. However, it didn't save and ignored the invalid input.
I feel in additional to UI type validation, we might need to add server side field data type validation in order to prevent such behavior.
A Nice to have would be to take also in account the setting in CiviCRM (language, currency and localization), on the decimal character. In French, the decimal character is the comma and in english it is the dot.