What is the best way to validate

Hi together,
not sure if its a question or a discussion.
i just discovered that isnull() for a nested CDT is not working properly

I have a CDT:  

nested CDT "attachment"   {"id"(integer), "document"(document)....}

in CDT Processdata {"id"(integer), "startedAt"(datetime), "attachment"(CDT attachment),...}

-> If I try to do isnull(ri!processData.attachment.document) it is not working and i always get false as result, even the value is NULL (100%)
-> if I try      ri!processData.attachment.document= todocument(Null)   its fine.

->  isnull(ri!process.documenttest) is working, so it seems to be just an issue regarding nested document CDT as far is can clarify.

a) is that a bug by appian?

b) what would be your approach to solve it? What is your best practice here?

Thank you all :)

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