Well it is a rather unique issue but is not a bug or error on the part of Unreal 4/5. Sticking to a possible reason with in context.
My animation weapon of choice is Motionbuilder as it’s designed to handle animation data in bulk takes rather then individual time lines in say a 3ds Max or Maya. As an app it’s design to handle animation more on the data level rather on the use of tools available for individual animation keyframeing.
I mentioned the above as unlike “do every thing well” applications MB by comparison handles a mountain of animation takes that could be hundreds contained within in a single file. Kubold for examples supplies all of his animation includes the MB source file in which he used to compile and export all of his animations to FBX format.
OK so how an invalid or null frame could be included, as the OP mentioned seems to occurred with purchased assets on all of the animations, is the process for export requires all of the keyframes applied to the control rig needs to be plotted down to the base Epic rig. As mentioned this could be hundreds of takes so a batch process is used to plot the frames based on the visible timeline so if the number of keyed frames is 10 but the timeline is 12 then the additional 2 frames are still plotted as part of the process. In Motionbuilder speak these would be consider null or invalid as they should not be considered as part of the intended batch process but rather over looked as part of the batch processing with out consideration as to individual requirements.
Once plotted down the take set is ready for export to FBX as the take can be exported based on it naming convention so is batched process with out individual verification as part of the process required by such programs like Maya 3ds Max or even Blender.
Since the OP did state it does seem to happen with purchased assets, and knowing how a lot of the packages were consolidated, I’m “assuming” the error originated in the original process and not a bug in UE4 as animation once imported is absolute data.
Motionbuilder does have the habit of small details being overlooked but due to the shear amount of data it’s easy to overlook small details as I’ve done so more that on one occasion