Q: Are there any changes for BM in 7.3? I thought there was a 10% buff?

No. In 7.2.5, our overall damage was given a flat 6% increase as the “Beast Mastery spec.” In a PTR build for 7.3, 4% was added to several abilities, but has since been reverted.

Patch notes for 7.2.5: http://www.mmo-champion.com/content/6687-World-of-Warcraft-7-2-5-Patch-Notes

Patch 7.3 PTR - Build 24727 (2017-08-01) added a 4% increase:

Beast Mastery

Beast Mastery Hunter Increases damage/healing of Barrage, Chimaera
Shot, Cobra Shot, Kill Command, Multi-Shot, Stomp, Volley by -6%-
(10%). Increases periodic damage/healing of Barrage, Chimaera Shot,
Cobra Shot, Kill Command, Multi-Shot, Stomp, Volley by -6%- (10%).
Increases damage/healing of Bestial Wrath by -6- (10). Decreases
damage/healing of A Murder of Crows by 45. Increases
damage/healing of Barrage by 100%. Increases damage/healing of
Volley by 50%. Hunter - Beast Mastery Spec.

Source: http://www.mmo-champion.com/content/6853-Patch-7-2-5-Hotfixes-August-1-Patch-7-3-PTR-Build-24727

Patch 7.3 PTR - Build 24853 (2017-08-18) reverted the 4% increase:

Beast Mastery

Beast Mastery Hunter Increases damage/healing of Barrage, Chimaera
Shot, Cobra Shot, Kill Command, Multi-Shot, Stomp, Volley by -10%-
(6%). Increases periodic damage/healing of Barrage, Chimaera Shot,
Cobra Shot, Kill Command, Multi-Shot, Stomp, Volley by -10%- (6%).
Increases damage/healing of Bestial Wrath by -10- (6).

Source: http://www.mmo-champion.com/content/6908-Patch-7-3-PTR-Build-24853-Dressing-Room-Update-Blue-Posts-DLC-594

Additionally, someone on the internet datamined and believes there was a 6% damage nerf to Stomp. This has not been confirmed, and is probably an inconsequential amount that no one will actually notice.