When we reopen the issue after Resolving the issue
the resolution won't be "Unset" or "Unresolved",
but we can add post-function to clear the resolution by How to Edit the Resolution of an Issue .

The problem rises when the SLA of ticket has already stopped when we have resolved it already
and by reopening it, it won't continue from the time we have resolved it.
As there is no option at Service Desk >> SLA >> Edit >> Start (Begin counting time when) to set the resolution to "Unset" or "Unresolved".