I tested with priority 5 and 20 and it made no difference.
I used the hook acf/validate_save_post to output $_POST in the console. Then I saved a the master again with a new child. But I can’t see a kurs_id field in the output.
Then I did the same but gave the kurs_id field a default value of 0. Now I can see a the field in the $_POST with the default value. But in the end, this did not solve the problem either.
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