I can't answer your question, but I prefer to use my LG TV remote to control everything and have the TV turn the Bose on and off. You might want to try that. The only time I use my Bose remote is to turn off the Bose after I have been using it to play music from my iPhone.
Thank you for posting and welcome to the Bose Community.
The Soundbar 500 should only default to the last source played when you turn it back on.
Can you confirm that the last source you were using when this happened was the TV source?
Let us know the results.
Joel - Community Support