FBX reimport issue with Interchange Framework

A licensee has reported that the following repro steps can result in a "Failed to merge bones" error:

• One of our Skeletal Mesh assets was imported into engine version 5.4 using an older version of the FBX importer.
• The engine was subsequently upgraded to version 5.5.
• The art team then modified the original FBX file, adding new bones.
• Attempting to reimport it using the reimport function resulted in the following error: "Failed to merge bones"

Internal only: a repro project has been provided. See repro steps for a faster repro.

Known workaround

Set the console variable "Interchange.FeatureFlags.Import.FBX" to false to use legacy FBX import, instead of Interchange.

Steps to Reproduce

• Launch editor
• Set "Interchange.FeatureFlags.Import.FBX" console variable to false
• Drag and drop attached UI_Test.fbx file into content browser.
• Check 'Skeletal mesh' when dialog pops up
• Import
• Set "Interchange.FeatureFlags.Import.FBX" console variable to true
• Reimport newly created skeletal mesh with new file using attached UI_test2fbx.fbx file

Actual Result: Dialog pops up stating "Failed to merge bones". Selecting 'Yes' or 'No' does not result in the same hierarchy as a reimport done with Legacy.

Expected result: No dialog popping up. Asset is reimported as with legacy.