This one is a bit hard to explain (Non-English speaker here) but I'll try.
Well you uses Mdlvis to added a new animation slot (I added a new one with the number started with 100833) and copy-paste every frames from the original buggy animation slot (In this case, 6833) to the new one (100833).
You can't add a frame numbers (For example: 1000) that's smaller than the previous numbers (For example: 8000) after you have the 8000 frames one, otherwise the 1000 frames animation won't work in-game and only can be seen in Magos (Model Editor). This is the problem that your model have.