The problem is that UAT is on a preview release and SIT is not. A preview release is a version of Salesforce that contains the features and enhancements that will be available in the next major release. Preview releases are usually available a few weeks before the official release date, and they are assigned to a subset of instances. If UAT and SIT are on different instances, they may have different release versions, which can cause deployment issues. For example, if UAT has a preview release that contains some new metadata types that are not available in SIT, then deploying those metadata types from SIT to UAT will fail. To avoid this problem, the architect should check the Sandbox Preview Guide1 and the Release Notes2 to see if there are any changes that affect the deployment, and refresh and redeploy the sandboxes accordingly. There is a problem, and it is not expected behavior. Using the DX command line instead will not solve the problem, as it will still encounter the same version mismatch. SIT is not on a preview release and UAT is not, as that would not cause any deployment issues.