Not sure if this is a bug or just hasn't been added yet, but the Scimitar Dreadnought doesn't "implode" properly when it's destroyed. The singularity breach animation plays as normal, but the ship just sits there doing nothing until the animation ends (after which is just disappears).
The ships imploding effect is just a recent addition, prior to this Romulan ships just did the exploding death roll like every other ship.
Remember the Scimitar in the movie exploded because Data fired into it's thalaron device detonating it, not because it's energy core imploded.
Does the scimitar even have a singularity core?
It could be that they simply haven't finished visuals for the ship since it isn't a part of the 0 to 20 sequence in Romulan content. There are still many ships yet to be added to Tribble that they are currently using the T'liss as a placeholder for.
I can't wait to see what the Ha'apax is going to look like finished.