Created attachment 187420 [details]
A before and after example of copying eight keyframes with Discrete interpolation and pasting them.

SUMMARY
After copying and pasting keyframe(s) on the same Effect timeline, the pasted keyframes' interpolations are reset to Linear automatically.

STEPS TO REPRODUCE
1. Make a new project
2. Add a video clip
3. Add an Effect to the video clip
4. Add a keyframe to the Effect timeline
5. Set the keyframe's interpolation to anything other than Linear
6. Copy the keyframe
7. Paste the keyframe onto the same timeline

OBSERVED RESULT
After copying and pasting a keyframe or set of keyframes with Discrete interpolation, or any other interpolation that is not Linear, the pasted keyframes have the Linear interpolation instead.

EXPECTED RESULT
After copying and pasting a keyframe or set of keyframes with Discrete interpolation, or any other interpolation that is not Linear, the pasted keyframes keep the interpolation that the initial copied keyframes have.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux Mint 22 "Wilma" (base: Ubuntu 24.04 noble)

ADDITIONAL INFORMATION
The bug also occurs if you copy a keyframe and paste it onto a different timeline of the same Effect type, including timelines on other video clips. However, the dropdown option to import copied keyframes onto a different Effect will keep the interpolation of each copied keyframe as is, instead of resetting them. (Import Keyframes from Clipboard...)