A lot of people with soundbars; specifically Sonos, have reported that normal 5.1 content appeared to be way more quiet/lifeless than Atmos content.
The solution on some systems was to disable eARC and have the TV recode the signal to DD+, or to force the Apple TV to Dolby Digital for non Atmos content, for it to sound like it was intended to sound.
People were also complaining that the internal apps on their TV sounded better for non atmos content, than the Apple TV.

26.4 beta 1 introduces a new function called: 'Continuous Audio Connection'
'AppleTV uses a Dolby MAT connection for glitch-free playback across formats. Older receivers may indicate an Atmos connection, but original mixes will not be modified.

With this enabled I can confirm that it fixes the issue.

Gear: Apple TV 4k (3rd gen) - LG G4 - Sonos Beam gen 2 with era 100 surround and sub mini.