@YodAI , the problem you experience can be explained by your expression relying on auto-determination of the paired items (via .item.). This could fail when the workflow includes IF nodes, switches, and custom nodes. That is when one-to-one relationship is deviated.
You can refer to the explanation I recently provided in Some time IF Node when it get Else next nodes dont see previous Nodes - #3 by ihortom. The doc with further guidance on referencing items in the previous nodes is Output of other nodes | n8n Docs.
By selecting the right reference expression you can make your data processing in the workflow reliable, no need for a workaround with a “temp data store”.