If you are connecting through the AVR, connect the Roku directly to the TV (with ARC to the AVR) and see if the issue persists.
Or vice versa if connecting to the TV - connect through the AVR instead.
Make sure you enable CEC/verify any lipsync settings in the AVR.
Make sure you enable BYPASS when enabling AV Sync adjustment in the LG.
FYI, LG has a history of AV sync issues (specifically, delayed audio) in every model year for years, which they are continuously attempting to address with firmware updates.
You should probably refresh the HDMI/CEC (including lipsync) handshakes: power everything off for several minutes, then power your AVR completely on, then power your TV completely on, then power your Roku on.