Main process not continuing after sub-process completion (intermittent issue)

I am facing an intermittent issue with sub-process execution in Appian.

A sub-process completes successfully and, in the parent process, the sub-process node shows as completed. However, the main process does not advance to the next node and remains stuck at that point.

This is happening both in multiple process models and at different points in the flow.

The same process models have executed successfully in other test runs, including cases where the same sub-process followed the same Flow and completed correctly. There are no errors logged, and the sub-process always completes correctly.

The only common pattern I have noticed is that the issue tends to happen in process instances that have been running for a long time and have traversed many nodes and multiple possible paths before reaching the sub-process.

Has anyone encountered a similar behavior?

Are there known engine limitations or considerations (e.g. long-running process instances, high node count, memory/state handling, or sub-process configuration) that could cause the parent process not to continue after a sub-process completes?

Any guidance on what to check or how to further troubleshoot this would be appreciated.