one reason could be you would have error messages on clipboard, this could be of any reason saying you have some validations on the current flow action and you would not showing that page messages on the screen. So you are unaware of the reason.
We have enable to move back and forth even with errors. this behaviour seen in few cases recently not all the cases. We are unble to find the root cause. In the trace it shows in GoToPreviousTask activity after executing 8th step it jumps to 19th step.
Activities we used to go back and next are ootb final rules
If step 7 of GoToPreviousTask is obtaining a value back from determineDeferErrors that seems wrong to you, I suggest you use pega tracer and enabled all event types and all rulesets.
Examine the step page, primary page, and the parameters passed to determineDeferErrors, as well as the properties on those pages, particularly the properties looked at by the determineDeferErrors java, which you should study.
Hopefully, that will enlighten the situation. /Eric