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
Form for editing records
Form for creating and editing records
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.
How to not loose the previous configuration
All forms described above operate independently. If you revert to the previous settings in the Create & Expand Records section, your previous child form(s) will be restored. You can then copy and paste the configuration between the old and new child forms, as described below, so you won't loose your work:
If you haven't done so already, revert your settings in the Create & Expand Records section to their previous state.
Open the child form
Under Options (at the top), select Copy
Change the settings in the Create & Expand Records section as needed
Open the new child form
Under Options, select Paste
Copying the configuration from one child form to the other will also permanently link any existing grandchild forms (i.e. child forms within the child form) to both child forms simultaneously, meaning that any changes made to these grandchild forms going forward will then apply to both the currently used, and previously used child forms (e.g. the child form you copied from, and the one you pasted to). If you do not plan to use the previous child forms going forward, this may not be relevant at all, but it is something to keep in mind if you ever switch back to the previous configuration.