I think this is due to the fact that there is no way to synchronize between when the capture occurs and weather or not the amvideocap0 buffer is completely full of data.
I wrote this script that basically replicates the capture code Kodi uses but in python, it preforms 3 captures one after the other, guaranteeing that the last one will contain a full frame of data.
Give it a go if you want. As an added bonus it captures just the video frame, no GUI overlaid on top.
script.amlogic.framecapture.zip (3.7 KB)
Just add the script to any button on you remote using a keymap and you are set.
<keymap>
<fullscreenvideo>
<keyboard>
<key id="your_key_id">runscript(script.amlogic.framecapture)</key>
</keyboard>
</fullscreenvideo>
</keymap>