Looks like the flow may be having some glitches with the list schema, where the field was not marked as mandatory.
Please see if you can give a try with following steps:
a. Take a back up of your current flow with Save As/Copy to have your copy.
b. In the original flow, remove the Update List Item action.
c. Save the flow and navigate out of the flow by clicking My Flows.
d. Edit your flow again. Now, add Update List Item action.
e. This time, you will see all the fields properly including Required fields.
f. Now, include all your changes in the Update List Item action and save the flow & run it.
Please let me know if the above steps helps you.