The morphed summon has same stats as the main summon in your grimoire. What doesn’t get transferred over is the additional DMG from your INT from your summoning skill, so you will notice your morphed summon is weaker than your original summon because it acts as if you had 0 INT.
You can test this yourself by using a different, weaker card as your main and then morphing your one star temple shooter. It will do weaker damage than if you morphed your 10 star templeshooter into the 1 star templeshooter, showing that the stats are still maintained.
Unsure if it’s a bug, or if it’s intended since no word from IMC when this issue was reported months ago.