Can LC filter at the input of the switching regulator cause problem?
I've never had problems with LC filters in front of DC/DC converters even with power hungry loads.
Time ago I designed 3 boards. Each board had DC/DC converter (*) and a GSM module which drew pulsed currents of 2 A and pulse width of 577 micro seconds.
I had an LC filter just like you in front of the DC/DC converter. L was 151 micro Henry and C was 220 micro Farad.
They are still running on the field after 15 years of work.
(*) Texas Instruments' LM2575, LM2576, LM2596
Modern DC/DC converters switch at high frequencies and unless that chip has a silicon bug, I don't think that the problem is the LC filter.

Add a battery powered circuit made of:
1 SR flip-flop
1 Schmitt Trigger inverter
1 Led

Design the circuit in a way that, when the 18 VDC disappears, the led is turned off.