Yes that is allowed. It's almost impossible that this will not happen. Due to the heat from the soldering iron, the insulation always melts slightly backwards …..unless you use a heat shunt (heat sink) or a heat-resistant insulation (such as Kapton/Teflon). The insulation may melt but not burn / char. For detailed pictures see IPC-A-610 or the IPC/WHMA-A-620 standard.