LC ringing on synchronous buck converter
Yes, this is perfectly normal. The LTC3891 switches off the lower MOSFET when it detects that the coil current is reversing, which leaves that node open-circuit. The residual energy in the coil and node capacitance at that point drives the ringing that you see. The ringing does no real harm, except that it can get into other places where you might not want it, including radiated EMI. This can sometimes cause you to fail an EMC test.

Putting the chip into "forced continuous operation" will leave the lower MOSFET switched on whenever the upper one is switched off. Among other things, this will completely eliminate that ringing, at the expense of slightly reduced overall efficiency. This is described at the bottom of page 12 in the datasheet.