Found while testing MCPE-152993 and closely relates to MC-190053 and MCPE-34085.

The bug

Unlike water, lava does not have any interior planes corresponding to the outside six.

How to reproduce

1. Recreate the setup shown in the screenshots attached
2. Enter the lava such that it isn't foggy, as to make any inside planes clearly visible

Expected results

There would be planes inside of the lava cuboid.

Actual results

There are absolutely none.