Answer
Treat REST API integrations and IFS-Connect-based integrations as architecturally different rather than expecting the same monitoring model: a REST API call that fails returns an HTTP 400-style error with exception details directly to the caller, so it's the calling system's own responsibility to detect and reprocess the failure, similar to working against any external SDK-style API. Inbound integrations still routed through IFS Connect (via routing rules/addresses calling integration projections or PL/SQL methods, as a modern replacement for old inbound BizAPIs) do still get the familiar Application Messages-based observability and reprocessing — so the presence or absence of that reprocessing model is really a consequence of which integration path you choose, not a general platform limitation.