Looks like Auto Exposure gets kicked in when playing.
You probably have a custom exposure set in the Editor Viewport instead of using Game Settings like the screenshot below.
UE5.6
Instead you can adjust it using a Post Process Volume
Enable Infinite Extent to affect the whole scene
In the Details Panel of the Post Process Volume, Under Lens → Exposure, you can either set both Min EV100 and Max EV100 to the same value (like 0 for example) to turn off Auto Exposure. Or you can set the Metering Mode to Manual (You can also turn it off for the whole project in the Project Settings → Engine → Rendering → Default Settings → Auto Exposure).
You can then use the Exposure Compensation Slider to adjust the Exposure.