Creating a 257 GB AVI file just to verify this report would be somewhat difficult for the
zero active Avisynth+ developers so don't expect any updates on this end.
Besides, this looks more like an FFmpeg issue. FFmpeg would be able to handle over a hundred million RIFF chunks if only
one line was changed.
The OpenDML spec doesn't seem to indicate any upper bound on the number of chunks, but I didn't bother reading through it very carefully. It would be pretty weird if there was an artificial 256-chunk limit, though.