A workflow has two RPA activities, one is executed in Robot Group1 and the other one is executed in Robot Group2. When the first activity ends, the robot in Group1 remains "IN USE" until the workflow has ended completely. Once the first activity completes, the robot status is still in in_use (running) state. It takes some time for the robot to update its state from in_use to ready. The reason behind the status of the robot "IN_USE" is because the robot is reserved until the entire flow completes. This is because in some cases for a flow it may be required to run two or more activities on same robot group. For instance, if the robot is not reserved until the flow completes, then there is a chance that some other activities belonging to other flows will occupy this particular robot. So the flow execution becomes slower in this case.
This can be changed on the central UI,
Got to system configuration -> system settings and then uncheck "preserve robot's session", click on save. After you do this, the robot will show "READY" status once the activity assigned to it completes. It won't wait for the other activities of the same flow. So making such change only if the user knows the further executions are not going to be in the same robot.