Hum, If you can’t get a root motion montage to overly onto a regular animation sans root motion enabled then your asset is likely at fault.
It could really be due to a lot of things, the first thing you need to check is that the skeleton referenced is the same.
The editor would/should/could usually throw a fit if you attempt to mix/match skeletons when layering animations, but it could also not.
So, if your skeleton is the same on both animations, and the mesh is rigged correctly (which you would know if it’s not as it would decompose in all the animations you imported so far), then there’s one more option that may have been used to make the skeletons match and that’s Re-targeting.
If someone re-targeted the imported animation to the correct skeleton to get this to work, the re-targeting may be bad/the issue.
I would suggest re-importing the base mesh - sans animations.
This will create a new skeleton/asset - you can use the same materials and even replace the old PHAT if you have modified it - but for testing, we just really need to know the skeleton is clean.
Then import the animations
make sure to select the New skeleton during import and only add in the animations.
Once that’s done, you are 100% sure stuff is clean.
you should right away create a blank anim bp and test the montage slot situation.
You can right away just layerblend the 2 poses into the final pose, without any need for input or anything. Persona will compose the 2 and if any explosion/bloating happens you’ll see it instantly in the little preview window.
Now, assuming that works, the next step is to test it out by replacing one of the 2 animations to one that has Root Motion and the appropriately checked check-boxes for it.
I tried this in my current project - with just a simple “slide that root bone” animation, and the reloading animations/firing/etc. all play fine (though replication doesn’t work since it’s root motion).