while waiting for experts to respond, allow me to speculate... You mention "group of task record" -- if that implies task group, tasks which are members of a group may be interdependant/unmovable unless as a group unlike individually added tasks. Have you tried altering sequence without cancelling? If my speculation is correct, you shouldn't be able to do that either; this kind of behaviour might, or should, be configurable in task group definition.
you did not mention the version
In my example it is a group individually Added tasks but it also applied to the one added in task group. You can altering the sequence without any problem, the issue occurred when you cancelled one, you are not able to move up task that are below the one cancelled. It happens frequently that we cancel some tasks because they are not applicable for the fulfillment of the request, for that we move the task to the last sequence and then cancel it so that it won't blocked the other to be moved up.
We are in ARS and ITSM 18.05
Server group Environment