See example where it will return items from List A (Issues Register List) that don't exist in List B (Issues). I'm comparing List A internal ID field with List B Issue Id field. I'm not sure what you have but should give you what you need in terms of logic.
List A (contains the full list of items)
List B (only contains some of the items)
Below is the full flow. I'll go into each of the actions.
The first thing it does is get all the items from both lists.
Select Issues IDs get a list of all the Issue Ids into an array using the following expression. Note that it also converts the values to int.
int(item()?['IssueId'])
In my case this will result in the following array.
We then use Filter array to check if the items from Select Issues IDs do not contain the ID from List A. This will result in the Filter array only containing items from List A that are NOT in List B (based on the ids).
value comes from Get items Issues Register
Output comes from Select Issues IDs
ID is item()?['ID']
We can then use an Apply to each to iterate over the items in the Filter array and add what we want to List B. In my case I'm just adding the Title and ID from the items in the Filter array.
The final result of List B (Issues) would be:
