When you pass a record from PB to Flow said record only contains fields that have values (not null). Otherwise those fields are "not set". You should check if they are not set first, then check if is null;

Use the was set operator and your conditions could look like this

{!ParentAccount.CSM__c}   was set   {!$GlobalCOnstant.FALSE}
{!ParentAccount.CSM__c}   is null   {!$GlobalCOnstant.TRUE}

with All conditions must be true (OR)

When you pass a record from PB to Flow said record only contains fields that have values (not null). Otherwise those fields are "not set". You should check if they are not set first, then check if is null;

Use the was set operator and your conditions could look like this

{!ParentAccount.CSM__c}   was set   {!$GlobalCOnstant.FALSE}
{!ParentAccount.CSM__c}   is null   {!$GlobalCOnstant.TRUE}

with All conditions must be true (OR)

Change your Decision Element to look like this:

Note that the only difference is this: