When using classic SLA, if a template has a service type and a new ticket is created with the template, the service type attached to the ticket is added but if the ticket is re categorized having a service type, it will not attached to the ticket.
This behavior is expected if the template was created before the classic SLA option was installed, this causes that the template is not valid to be used as it handles the Service Types differently when the option is or not installed, with classic SLA, the table call_req handles the service types data without classic SLA the table attached_sla handles the service types data.
Service Desk 14.1 or higher
If Classic SLA is installed, all previous ticket templates will no longer be valid, it is necessary to create new templates with the required Service Types in order to Service Desk to understand the fields that need to be used. the same will apply if the ticket templates were created to use Classic_SLA and the option is uninstalled.