Mine still pops every time I start it up...does not matter if I wait for the full cycle of the start up or if I just crank the key right off the rip.

Whatever it is...my buddy has a 2019 SL450 and his does not do it. Same exact stereo other than new Vision Control screens that are on the 2019s.

When I had mine in for the 100 hour service I asked them to check and upgrade any firmware updates on the system and there was nothing available.