AWS SQS FIFO queue behaviour with AWS Lambda reserved concurrency

So I have 10 message groups in FIFO queue having 2 messages per group and I have also reserved lambda concurrency set to 5. (Lambda completes execution in 1 min and SQS visibility timeout set to 2 mins)

when all 20 messages are pushed to queue, SQS inflight messages gets set to 10 and then after the execution time, 5 messages gets processed successfully and other 5 moves to DLQ.

And then the next executions inflight messages gets set to 5 (as reserved lambda concurrency set to 5.) and processes as expected (This should be the expected behaviour right?)

Any particular reason why this is happening?

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