5E rules regarding armor and being prone [closed]

In 5E, the rules seem pretty straightforward, but clear, regarding armor and the associated AC. However, it seems a little out of balance for characters who fall prone.

Does, or should, the AC for a character change if they are prone? For example, the Paladin in my group I am DM’ing was wearing plate armor and was knocked unconscious, falling prone. As far as I can tell, this means he still retains his 18 AC, regardless of the fact he is no longer moving.

Should the AC stay at 18 or get reduced? Am I missing where this might be covered?