Hey all!
I have an issue that I can't figure out at all. 
I have a simple flow. It takes an Order record as an input, finds the related OrderItems, and loops through them to update a record on them with a currency value. 

But, when I run the flow, the Update Record node is failing. The Loop collection variable looks like it has the right collection, but the Update tries to "update records: null", which makes the Flow loop endlessly and eventuallly hit max flow depth. 

The Flow is invoked by a Process, when the status of the order changes. 

The Flow debug log is as follows: 

FAST LOOKUP: Get_NS_OrderItems
Find all OrderItem records where:
Product2Id Does not equal {!FTK_Prod2Id} (01t2M000005o9eQQAQ)
Serialized_Item__c Equals false
OrderId Equals {!Active_Order.Id} (8012M000008Po5cQAC)
Store the values of these fields in Get_NS_OrderItems: Unit_COGS__c, Product2Id, Id
Result
Successfully found records.

LOOP: NS_Loop
Loop Through: [8022M00000CUp3SQAT]
Iteration: 0
Current iteration item: 8022M00000CUp3SQAT

FAST LOOKUP: Get_NS_Asset_COGS
Find all Asset records where:
Product2Id Equals {!NS_Loop.Product2Id} (01t2M000005oOVYQA2)
Store the values of these fields in Get_NS_Asset_COGS: Unit_COGS__c, Id
Result
Successfully found records.

ASSIGNMENT: Assign_COGS_to_OrderItem
{!NS_Loop.Unit_COGS__c} Equals {!Get_NS_Asset_COGS.Unit_COGS__c}
Result
{!NS_Loop.Unit_COGS__c} = "11.48"

UPDATE RECORDS: Commit_NS_COGS
Find all OrderItem records where:
Id Equals {!NS_Loop.Id} (8022M00000CUp3SQAT)
Update the records’ field values.
Unit_COGS__c = {!NS_Loop.Unit_COGS__c} (11.48)
Result
Failed to update records that meet the filter criteria.

The error message is here: 
Error Occurred: The flow tried to update these records: null. This error occurred: CANNOT_EXECUTE_FLOW_TRIGGER: We can't save this record because the “Write COGS to Order Products” process failed

Unfortunately, I've already reached out to SF Support and they cannot help, as Flows support is not included in our current support package.