1. What happened

HDRP Light brighness is being animated correctly in editor but not when played in player.

2. How we can reproduce it using the example you attached

1 - Add a Light in a scene
2 - Create a timeline that references this light in an animation track
3- Go into record mode and add keys for the brighness using the inspector, make brighness vary.
4- Play in Editor: Light is animated correctly
5- Make a build of a player (ex: PC), then play: Light does not animate.

Notes:
The timeline seems to access the member displayLightIntensity which does not update the light component, the correct use was to use the accessor intensity which performs the light update. (btw this stored value is part of a set of members that should be private serialized instead of public)

There seems to be a workaround to force the update of the light but it is enclosed in #ifdef UNITY_EDITOR so this behavior seems expectted, yet wrong.
TimelineLightB