Jerky playback on v18 with change of scaling algorithm

On v18, I’m playing a 720x480/23.98Hz h.264 mkv, decoding in software, with output resolution set to 1080p/23.98Hz.

If the upscaling algorithm is set to either nearest neighbour or bilinear, everything is okay; but if I select any other upscaling algorithm, playback becomes horribly jerky. CPU utilisation is quite low, so it doesn’t seem to be running out of CPU capacity (unless it’s mis-reporting that).