getIds { output: null }

I suspect there is a bug in the FireFly signer ABI encoding. This contract demonstrates the issue: https://mumbai.polygonscan.com/address/0xc0bc785b2c5494820B63c7108547b4652b9742aD#code

Steps to reproduce:

Create a new FireFly stack (using default settings with a local blockchain node is fine)
Create a contract interface / API with the contract listed above
Call the POST /invoke/createCoupon endpoint with the following payload:

{
"input": {
"_end": "1234",
"_id": "2345",
"_ipfsUrl": "3456",
"_start": "4567"
}
}

Call the POST /query/allCouponIds with an empty JSON object as the request body
Response:
We will get an response as : getIds { output: null }, even i tried with other functions receiving the same error.
On Polygonscan I see a valid response from this method.