I have a parallel action in my list workflow which will update a field (Log). I have 4 parallel actions which will run whenever an item is modified, check for different conditions and update a single field (Log).
Now when the workflow enters parallel action and conditions are true in 3 of the parallel branches and update is to be done. On my workflow history the update item action looks green and workflow completes successfully but it wouldn't have updated the column in sharepoint.
As i observe it will update the Log field in only one branch of the workflow in SP column.
Instead of updating the item in a parallel manner it would be more robust to set variables in your parallel task and afterwards just have one update item action pushing all the variables to your item.
However you can try to fix your issue by adding commit pending changes right before your update item actions.
This will ensure that all other update actions will be saved to the database before you update the item again. Maybe it helps.