How to get 50 Hz sine wave from inverter bridge driving PWM pins of MCU
Try filtering separately, only H1, for example. If you get results, L1 should give the 180 degrees version, while H2 and L2 should give similar results. This looks like a 3-level PWM, so coupling H1 to H2 with the RC filters, and L1 to L2 should give you the two point differential filtering necessary to reconstruct your signal, V(H1_H2) - V(L1_L2). Here's a quick'n'dirty simulation showing what you tried and what I mentioned (please note that I am making an assumption about the PWM, I don't know what you have there):