Solidity state variable resets to default inside function - blockchain

Question

For this project, I am coding smart contracts for Ethereum using Solidity.
In the following code, the variable numCertificates should initially be 1. Nonetheless, newCertificateId is 0 when assigned. According to the debugger (I'm using remix.ethereum.org), for some reason, numCertificates becomes 0 as soon as the line where newCertificateId is assigned is reached.
I have played around with remix a lot, and according to it, numCertificates is 1 until exactly the aforementioned line is reached. Also, I think that if I modify numCertificates in another function, the variable stays the same as before.
My attempts at trying to figure this out lead me to believe that when I access numCertificates, I am not accessing the public state variable but something else.
Why does this code have this problem and how can it be solved?
pragma solidity ^0.4.21;
contract MyToken {
// ... lots of irrelevant stuff here
mapping (uint64 => Certificate) public certificates;
uint64 numCertificates = 1;
// ... lots of irrelevant stuff here
function MyToken() public {
// ... likely irrelevant code
}
function produceCertificate(
// ... likely irrelevant parameters
) public {
// Create the certificate in memory
numCertificates;// only for debbuging
Certificate storage newCertificate = constructCertificate(meterId, timestamp, carbonMitigationValue, owner);
uint64 newCertificateId = numCertificates;
newCertificate.certificateId = newCertificateId;
// Save the certificate
certificates[newCertificateId] = newCertificate;
numCertificates++;
}
function constructCertificate(
uint64 meterId,
uint32 timestamp,
uint48 value,
address owner
) internal returns (Certificate storage newCertificate) {
newCertificate.meterId = meterId;
newCertificate.timestamp = timestamp;
newCertificate.value = value;
newCertificate.owners.push(owner);
newCertificate.ownershipSplit[owner] = value;
newCertificate.burned = false; // by default new certificates are fresh
return newCertificate;
}
}