This issue seems to have been reported and commented on for about four years now. Equipment and processes have all been different but the issue has been the same.
Mine started when I connected the mini-sub to my Arc SL. Not that it matters, but my audio comes from the e-arc port on a Samsung Q80T and the issue replicates across all of my inputs, including Nvidia Shield, SkyQ and native TV apps.
Prior to connecting the sub, for about a year the Arc retained the previous volume level it was set to when the TV was switched off/on.
I'm loathed to start disconnecting and reconnecting subs and soundbars because that's not what I paid for but after reading about this issue going on for a few years now, I fear won't have much choice.
