razor10 When the low-res textures are on spine while it fetches the high-res texture addressables, I see the image rendered incorrectly. But when enabled the "Straight Alpha Texture" on the material, such issues arent appearing anymore.

When creating the low-resolution placeholder assets, the original texture's texture import settings are copied. Did you see different properties set at the newly generated low-resolution texture than the original texture?

razor10 I'm on Linear colorspace & building for WebGL platform.

If you're using Linear color space, you must use Straight alpha workflow, using PMA will yield incorrect results. You should see warnings in the console and at the skeleton components.
See the documentation:
https://esotericsoftware.com/spine-unity-assets#Advanced-Premultiplied-vs-Straight-Alpha-Import

razor10 Is it because on-demand loader generates low-res textures with straight-alpha mode ?
How can I tackle this?

It's just the textures which are swapped at the material, no need to change any material settings.