Manually changing the related entity's form before saving the record does not auto populate the value in the mapped fields. If you save the record and then switch between the related entity's forms using the form selector the value in the mapped fields stay as is, however this cannot be achieved if you have a script which is designed to auto switch the form depending upon the records to be updated.

Repro:
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1. Create 2 test entities (Test A and Test B)
2. create custom field on both the entities (Field xyz)
3. create a new 1:N relationship between Test Entities A and B
4. open the N:1 relationship on Test B entity
5. go to mappings and map the custom field (Field xyz)
6. publish customization, save and close form
7. open Test A entity's form and insert a new sub-grid named Test B
8. provide value in the mapped field (Field xyz) in Test A entity, save the record and click on add new record in the related entity Test B and it auto populates the value in the mapped field (Field xyz)
9. create a new main form on Test B entity and switch the form without saving the newly created record in Test B.
10. This causes the values in the mapped field (Field xyz) disappear, if the record is saved before changing the form the value in the mapped field stays as is.