The GLTF importer adds an extra frame to the animation sequence when the source animation is slightly above the target length. As seen in the image below the source animation length is 40.02 which should be 20 frames long in ue5, but it is rounded up to 21 because of the .02 decimals.
This in turn means that there is an entire frame without any animation which looks like this:
This is the asset being imported:
worserobot.zip (2.8 MB)