Please find SPR 1129378 and vote on it.
SPR 1129378 - When creating a new process via an output, there is no way to write data from the new child process back to the parent
Explanation: This would in theory permit the closed loop record associations that are essential for our business needs. We have stage outputs that allow us to "Start New Process" where we can pass "This Record" (as an object field) to a field in the new process thereby establishing an association. However we currently are unable to "Update Process" from the new process back to the process which started the new process in order to update a similar object field with "This Record" values when the process is at an "END" stage even if selecting "ALWAYS" as the option and setting the field permissions to FULL CONTROL in the END stage.
ALWAYS should mean ALWAYS.
It's really a simple concept. But perhaps another option should be added like, "ALWAYS AND AT END STAGE".
This should be escalated to Extremely Urgent. It would really be nice to see this in the next service pack release. We are on 2020 SP3 now so PLEASE, PLEASE, put this in SP4.