Currently there is scarce research on the impact of grid forming controls on subsynchronous torsional interactions (SSTI) between synchronous generators and series-compensated lines. This work evaluates the impact of two main grid forming control variants on SSTI under different parameters and configurations. By means of small-signal analyses it is shown how the presence of power converters can impact the risk of SSTI and how this effect can be mitigated by a simple active damping strategy. Lastly, the small-signal conclusions are further verified by means of non-linear time-domain simulations.