I had the same problem. Samsung smart TV, Bose 900 soundbar and new roku ultra. I solved my issue and no more problems.
On the roku:
Streaming audio format set to Dolby
Digital output format set to passthrough
On the samsung smart tv:
Digital output on passthrough
This got rid of the constant Dolby Atmos showing on the sounbar. Now I get LPCM 2.0 on broadcast channels and either 5.1 or Atmos when expected, like Disney or Prime. Hope this helps.