Below is an example on how form breaks in domain separated instance:

1. Make sure you have only one ‘sys_ui_form’ record and in global domain:

2. Go to TOP/ACME domain, and create an incident (or open an existing incident that belongs to TOP/ACME domain)

3. Right click on the form header, select Configure à Form layout

4. Change the form layout from TOP/ACME domain and save it.

5. This creates a new ‘sys_ui_form’ and its related records in TOP/ACME domain overriding the ‘global’ domain ‘sys_ui_form’

6. Go to TOP domain and open any record like above which is created in TOP/ACME domain:

7. Configure form layout for this record from TOP domain: You will see the form broken.

Reason

This form exists in Global domain & TOP/ACME domain only, and TOP/ACME domain overrides the global domain.
When we have “glide.sys.domain.use_record_domain_for_data” property set to ‘true’, the record always opens in record’s domain. And when we have “glide.sys.domain.use_record_domain_for_processes” property set to ‘true’, the process (forms/form sections) on record’s domain gets applied as well.
From TOP domain, if you look at the ‘sys_ui_form’ list, you will only see the one from global domain. But this global domain process isn’t getting applied to the record in TOP/ACME domain. Hence the issue is caused.
If we set “glide.sys.domain.use_record_domain_for_processes” property is set to ‘false’ the above issue will not occur, but that’s not an actual solution.
As this issue is newly created in this instance only, importing the correct form/sections from other layouts is not an option.

Go to the ‘sys_ui_form’ list from global domain & expand domain scope.
Delete the TOP/ACME domain record.
From TOP domain create a new form layout for relevant TOP domain record’s and then override that from with its children like TOP/ACME by creating new form layouts from TOP/ACME domain.
Maintaining the hierarchy appropriately can avoid showing such issues [TOP overrides global, TOP/ACME overrides TOP]

If resolution 1 is not suitable, do not try to update the form layout from TOP domain that belongs to TOP/ACME records.
Go to TOP/ACME domain itself to update such form layouts of those records, as technically TOP domain doesn’t contain a layout itself.