If test failures are occurring in a sandbox but not in production and there have been no active code or metadata changes, it could be due to differences in the Salesforce release versions between the two environments.
Sandboxes can be on preview instances which get upgraded to new releases before production instances. This can lead to differences in behavior and could affect test executions. Ensuring that both the sandbox and production are on the same release can resolve such issues. API version differences and parallel testing settings typically do not cause discrepancies if the code hasn't changed, and while using SeeAllData=true is generally not recommended due to its dependence on the organization's data, it's unlikely to be the cause if the issue is related to the environment rather than the data.
References:
Sandbox Preview Instructions
Understand the Salesforce Release Process