Editor not properly handling frame ranges for 60fps animations

The editor does not seem to be handling frame ranges properly for 60fps animations.

Say you have a 200 frame animation that spans frame 0-200. Upon import it will be 201 frames. If you set the range to 10-200, the imported animation will be 181 frames (20 less instead of 10).

See repro for more detailed info.

The user actually provided pretty solid repro steps. See below

Must use a 3rd person template project. Import attached animation onto the mannequin skeleton.

I've slapped together a new 60fps fbx that should make it more obvious when the resulting frame range is wrong. Character can use the standard skeleton on import. The first 100 frames has the character sliding across the floor to a prep position. The second 100 frames has the character doing a flip to return to the starting point on the floor.

1) Import the entire frame range, resulting in 201 keyed frames. Play animation while viewing from the side.

2) [In the Anim Asset Details panel] Open up Reimport | Import Settings and select Set Range. Set Start Frame to 0 and End Frame to 201. Reimport. The asset is still 201 frames long and is OK.

3) Set Start Frame to 10. Reimport. The asset is now 181 frames long, shrinking by 20 instead of 10. The asset should be 191 frames but isn't.

4) Set Start Frame to 0 and End Frame to 50. Reimport. The asset is now 101 frames long and if played contains the entire first half (the prep frames) of the fbx. It should only contain the first 51 frames.

5) Set Start Frame to 100 and End Frame to 201 (the goal is for the asset to just include the flip). Reimport. UE4 4.7 & 4.8 Preview one will claim the reimport was successful. However reimport has silently failed and left you with the first 101 frames (from Step 4).

IF the Start Frame or End Frame is greater than 50% of the fbx frame count the reimport will fail. I believe this is because the user values in Start & End Frame are multiplied by 2 when processed. Every frame I trim from the fbx on reimport causes a loss of 2 frames. So setting a Start Frame to 100 on this fbx results in a zero frame import.

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