I've noticed some very odd behavior regarding the defense rating of a ship.
Whenever I'm in the blueprint design mode, defense rating of the ship seems to drop randomly. For example, I had a cruiser design with 81 DR, then when I removed the merculite missiles, changed fusion beam to graviton cannon or installed reinforced hull (WTF?!), the DR dropped to 18. After backtracking and doing the intended changes again, it remained at 81, but when I saved the design, it was showing only as 18... and after upgrading one of my cruisers to this new design, it also showed a DR of 18.

Is this a bug (I suspect so) or is there some game mechanism I overlooked? If it is a bug, I wonder if only the display is bugged or the incorrect number is also used during battles.

Got the same behavior here. I think it's a bug...

remember that the game is sitll in an early alpha state, these kind of bugs are common.
I think, but have no proof, that these kind of bugs don't effect combat though.
Something to also keep in mind is that the defense rating doesn't really say anything.
For example point defense weapons can give you quite abit of defense rating.
But of course they are worthless against beam weapons and torpedos.

I don't think it's in alpha state. It has left the development and is RTM.
The defense rating worked fine before the update to 49.3 and the behavior now is realy strange. When you are saving your own ship creations, the difference between the ratings while creating and after saving are realy great - sometimes over 100 pts. So it seem like a great bug to me.

just because they claim the game to be finished doesn't mean it is not in the state of an alpha.
The technology is fine (no crashes, not bugs), but the current features and small issues like this one are not even close to a finished produced.

I didn't mean my comment as an attack on the devs - I can absolutely understand that there are still bugs in the game.
I honestly simply wanted to know if this was a bug or there was some game mechanic I overlooked.

Anyway, I have sent a bug report about this using the in-game interface for bug reports.

What do you mean? I have version 49.4 1.2.31113 and I dont think it was updated today (patch notes also still say 09/15).
EDIT: I see that it has just been updated.
All clear now!