Each linked record field (or portal table) can have up to three separate child forms associated with it, each for a different purpose:
Form for creating records: Used when creating records is enabled, and editing is either disabled, or For creating and expanding (editing) records is set to Use different forms.
Form for editing records: Used when editing records is enabled, and creating records is either disabled, or For creating and expanding (editing) records is set to Use different forms.
Form for creating and editing records: Used when both creating and editing records are enabled, and For creating and expanding (editing) records is set to Use the same form.
Depending on your settings in the Create & Expand Records section, one or two of these three forms can be in use at any given time. Changing certain settings in this section will switch out these forms, which can seem like the form has been reset. For example:
Removing the ability to create or edit records: Disabling the toggles for creating or expanding records will result in a new child form being displayed, depending on the option you deselect. For example, if you disable creating new records, you'll see the child form that expands records; if you disable expanding records, you'll see the child form for creating records.
Going from using the same form to using different forms: If you change the setting from using the same form for both creating and editing records to using separate forms, the originally configured form will no longer appear. Instead you will now see both the form for creating records, and the form for editing records.
Fully removing a linked record field from the parent form: This will completely remove any child forms you had set up previously. This can only be reverted by reverting the parent form itself to its previous state, as explained here.
Unfortunately, it is not currently possible to carry over a form configuration when switching between these different forms, e.g. from only using a create form to using a 'create and edit' form, or to copy a create form's configuration over to an edit form. In these scenarios, you'd have to rebuild the child form.