This is just a guess since your files are not available, but I think you made the cut at the assembly level, so the actual part is still its full length. When you mirror the part, you thus get a full length part.
To fix this, you have two options:
1. mirror or recreate the cut again in the assembly to trim the part to the correct length.
2. Edit the part and cut it to the proper length. If you need different variations of part length, you'll need to save them as separate files, or make use of configurations.
Thank you Mr. for your answer