Issues with Proxies Made from H.265 Clips

I am having an issue with proxies created from H.265 clips recorded on a DJI drone. 

Playback in any timeline with clips linked to proxies is very slow and unresponsive, worse than using the original 4K clips. Clicking on another frame in the timeline takes consistently 15 seconds before the frame appears in the Program Monitor. Sometimes I will get the "Media Pending" screen before the first frame appears and it starts to play back. Playback will sometimes be smooth, but sometimes it will stutter or pause while the playhead moves in real time.

However, if I manually import the very same proxy clips separately into the project and drop them into a new timeline, playback and scrubbing are responsive and immediate.

Playback and scrubbing in the Source Monitor window is better, taking only a second or two before playing back, but still not as responsive as importing and playing the proxy clips directly.

This issue appears when proxies are generated as ProRes Medium Resolution, or as Cineform Medium Resolution. I am using a well-specced 2019 MacBook Pro with plenty of memory.

This does not happen on proxies created from clips that are NOT originally in H.265 format, including .MOVs recorded on Blackmagic cameras, or .MXFs recorded on Canon cameras.

I've cleared the Media Cache and reset Preferences.

Please let me know if you need any further details.