Please note that PowerApps has a data row limit for non-delegable queries set to 500 by default, which means only the first 500 records are retrieved if we cannot delegate a query to a back-end data source (filter, sort, etc.).
This value can be changed via PowerApps form > File > Settings > Advanced settings> Data row limit for non-delegable queries, up to 2000.
If the lookup column values do not show up completely, try changing this value to a larger number (2000, for example), then republish the PowerApps form and see the result.
Reference: PowerApps data row limit for non-delegable queries.
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