Hi Stefan,
Good Day,
Let me explain you the SLA calculation with respect to the OnHold feature in ServiceDesk Plus.
A technician in general puts a ticket onhold if he is awaiting for a response from the requester. During this time the SLA will not be calculated as the delay is from the user who has opened the request. As soon as the ticket changes from onhold status to another status which has the timer running the due by date will be recalculated according to the SLA configured on the ticket.
This is the current behaviour of the application.
Hope this helps.