eval isnull() does not work as expected

[eemhu]

Describe the bug

| where isnull(customerId)

returns rows with nulls, but if you try replacing those nulls using if statement, it doesnt work as expected

%dpl
index=sales_inventory earliest="01/01/2017:02:34:19"
| rex4j field=_raw "customerId=(?<customerId>\"([a-z0-9]{8}-[a-z0-9]{4}-[a-z0-9]{4}-[a-z0-9]{4}-[a-z0-9]{12})\")"
| eval new_field = if(isnull(customerId), "Null", customerId)

Expected behavior

How to reproduce

Screenshots

Software version

5.3.0

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Additional context