Please see if you have the following entries in IS log; then the 10.4.1 environment may need EBF-19160.
LM_36814 : Workflow [wf_Name]: Got scheduled request to start workflow.
LM_36813 : Workflow [wf_Name], run instance name  is scheduled to run now, schedule last saved by user [DELETED-550003071].
LM_36801 : Workflow [wf_Name]: Workflow will run next at [Mon Jan 01 00:00:00 1753].
LM_44260 : Workflow [wf_Name]: Workflow will be rescheduled after the current run completes.
LM_36318 : Workflow [wf_Name]: Execution succeeded.
Before you proceed with applying the EBF, please open a Support Ticket to confirm if the above EBF do not have any conflicts with an existing EBF (if there are any). Share the ebfHistory.info file with the Informatica Global Customer Support Engineer from all the nodes of the domain and the CSM output for review.
Thanks for your information. I will try to ask the Informatica support for EBF.
I believe when I saw the workflow log, there is log like that. unfortunately I didn't save the workflow log.
Currently I changed the scheduler to Run On Continuously, and didn't faced this issue. but still I'm curios why if change the scheduler to every 5min or 15min, sometime the workflow is look like unscheduled by themselves.
If it is indeed the bug that Syed is mentioning here , the workflow doesn't get unscheduled but actually gets scheduled to Mon Jan 01 00:00:00 1753.
Per my understanding there is a complicated logic as to when this happens that includes one run running longer than the interval between schedules.
If you do see Mon Jan 01 00:00:00 1753 in you IS log , please do follow steps mentioned by Syed above