The maximum number of rows returned by any SOQL over the REST API is 2.000 per request. In case your data source has more than 2k items the data will be cut off.
You can work around this limitation to request up to 12.000 items with the following workaround:

Original Data Source

Create a normal Salesforce data source and select the data you want to paginate over. For example:

https://xxx.salesforce.com/services/data/v45.0/query/?q=Select name from account
CODE

Paginated Data Source

Create a second data source of type Paginated Data Source and select the original Salesforce data source.

The pagination type here is called Follow URL because the URL of the next page will be returned with each response.

URL Selector

Define the location of URL for the next page

$.nextRecordsUrl
CODE

Result Selector

Define the location of all records for each page

$.records[*]
CODE

Finish Condition

Salesforce will send a follow URL until the last page. So select Empty Target as a finish condition

Finish Target

In case Salesforce doesn’t send a URL back we know its the last page and finish the pagination

$.nextRecordsUrl
CODE

Now testing the data source should return a valid result, which makes the data source ready to use.