If a task has actuals, but the ETC is down to 0 (either by time entry consuming the time or by marking the task as completed and then re-opening it), ETC cannot be added to it. The ETC revert back to 0 as soon as you click out of it. To enable editing, we have to open the task in Classic, add some ETC (e.g. 1 hr) and then come back to the new UX to update end date and ETC. Tasks without Actuals seems to work fine. (e.g. I can mark it complete, reset it to open and update ETC to some non-zero value)
This is working as expected with the current design in Modern UX and it works the same in Classic at the task level.
If you go into Classic UX and add the ETC column on the Task List and update the ETC there ; this is the same as if you go into the Modern UX on the Task Details Fly-out and update the ETC. This is the TASK-LEVEL ETC - it spreads the total amount proportionately over all assignments with respect to the rules outlined below.
The modern UX does not support assignment level ETC updates. The TASK LEVEL ETC that is entered on the Task Details Fly-out is spread over all assignments under the following rules (same as Classic):
The TASK ETC is NOT spread into an assignment with actuals and a ZERO ETC. (When there is ZERO ETC on the assignment, the Finish Date on the assignment goes up to the actuals thru date ; there is no remaining work expected for this resource assignment, therefore, no additional ETC is expected automatically. The only way to add more ETC on this type of assignment is to update the assignment directly which is currently not available in the modern UX)
The TASK ETC is spread into an assignment with actuals and a NON-ZERO ETC. (There is remaining work to be done and the Finish Date on the assignment goes past the actuals thru date and the total remaining non-zero ASSIGNMENT ETC can be influenced by the TASK ETC)
The TASK ETC is spread into an assignment WITHOUT actuals that either has a zero or non-zero ETC value - all the segments on the assignment is over-written by the Task level ETC spreading update.
NOTE: There are plans to implement a Tasks Assignment Grid. For the interim, the customer can create a CHANNEL on the Blueprint that exposes the Classic Task List for drilling into the Task Assignments.