There's a possible race condition when the flow is being read by a MID server and an update to the flow happens on the instance. This will cause the flow to error out. Additionally, there's another possibility that an update could happen on the on the instance after the flow has been sent back from the MID. Both scenarios can occur if the flow has an Ask For Approval with a group of approvers followed by an action that goes to the MID.
Steps to Reproduce
1) Create a flow with a group approval with multiple members followed by an action that requires mid.
2) Ensure the serialized process plan will span multiple sys_json_chunk rows by adding some steps with very large inputs
3) Execute the flow and approve all the approvals simultaneously
4) Hopefully the race condition will occur where the mid reads a partial JSON document for the plan.
The likelihood of the race condition occurring can be reduced by adding a "Wait for a duration of time" action between the group approval action(s) and action(s) that require mid. The the longer the duration of the "Wait for a duration of time" action the more effective it will be. 30 seconds may be sufficient, but you'll want to test this thoroughly in your environment.
Another possible workaround, would be to put either the approval or the action that goes to the MID in to a subflow and call the subflow via dynamic flows. As dynamic flows executes in a separate context this will isolate approval updates from MID execution. Dynamic flows is available in Orlando and beyond.
Related Problem: PRB1410951