Hello Folks,
I encountered the same issue and raised an OSS with SAP. SAP got back saying that this issue occurs when the IDocs are stuck in the "Transaction Recorded" status in SM58 due to processing of large amounts of tRFC calls. Please refer to note 1548446 to fix this issue. On a high Level, Make sure the Registration for the child system is active for both tRFC and qRFC in Output Scheduler (SMQS)
Hope this helps!
Thanks
Surakshith