I've seen this problem being reported all over "European" Samsung Forums.
When the Soundbar (doesn't matter which Samsung Soundbar as it seems) receives an eARC Signal it switches automatically to that Source from D.IN. This doesn't happen with ARC. So it seems to me that this is a Firmware/Software problem.
This is a huge problem for me as I use a record player on D.IN (optical in) and it switches to eARC after a few minutes.
I use a Sony TV which has Android and sends HDMI Handshakes in Standby (stupid I know) which causes the Source changes. But the same thing seems to be the problem with Samsung TV (the Frame for example) see here https://eu.community.samsung.com/t5/audio-video/q950t-soundbar-won-t-hold-d-in-input-when-earc-hdmi-...
Again, it doesn't happen if i set ARC on my TV instead of eARC but that isn't an option for me because I loose Atmos with this. Doesn't matter if CEC is on or off, I've tried everything (spend way to many hours on this).
So either make it possible for us to switch between ARC/eARC and D.IN or just make eARC act like ARC when it receives a Signal (don't change source from D.IN).
I'm asking the US Support team here because they seem to care more about the problem and may be in contact with the software team.