Happens in both HDRP and Universal.
Does not reproduce when in Gamma color space.
Reproduces in 5.7.1, 6.9.1, 7.0.1 and 7.1.1. Reproduces on SRP github project as of commit 73a37d1858 (checked on 9/18/2019).

Repro steps:
1. Use a project with HDRP
2. Create a new PBR Shader
3. Create a new game object, give that game object a volume component
4. Create a new post processing profile on the volume component, and click add override, and select "bloom"
5. Enable all options on the bloom and set intensity to 0.1
6. Open your PBR shader
7. Create a color node, set it to HDR, and plug it into emission
8. Make the color node 255 r, 0 g, 0 b, 255a, and set the intensity to 10
9. Save the shadergraph
10. Create a new material and assign your shadergraph to it
11. Add a sphere to the scene and add the material to it
12. Observe the level of bloom on the sphere
13. Go into the shadergraph and convert the color node to a property
14. Open the property and confirm that the values are the same you set earlier
15. Save the shadergraph
16. Observe the level of bloom on the sphere

Expected result: the level of bloom should be the same
Actual result: there is way more bloom on the sphere when the color node is used inline as opposed to property

Note: when in gamma color space, the bloom for both inline and property of the same color closely matches the bloom for a color property in linear, which leads me to believe that the inline color node is producing incorrect results.