ffmpeg Build Differences
Windows binaries are often compiled with libav or specific optimizations, whereas on Xubuntu, ffmpeg is compiled with native GNU/Linux libraries. These compilation differences can alter the behavior of the filters.Handling Input GIFs
A GIF is already a discrete sequence of images. On Windows, ffmpeg can interpret frames as still images with an implicit frame rate, which distorts the logic of mod(n,2) or mod(n,3). On Xubuntu, the interpretation is stricter: each frame is counted and filtered.Frame Indexing Issues
The counter n used in select='mod(n,2)' depends on how ffmpeg indexes the images. Windows can start counting from zero or reset based on segments, which retains more frames. Xubuntu follows strict linear numbering.
From @sawsenbenkhadhr