I noticed that the video elements in my recent post were not showing a thumbnail by default on iOS Safari.

Upon googling, found that this is a known issue. Luckily, there is a simple but kinda ugly fix for this by appending #t=0.001 to the src tag of source elements which forces the initial frame(s) to load:

<video>
    <source src="videos/rickroll.mp4#t=0.001" type="video/mp4">
</video>

And that did the trick.