When using the /summon command, the ender dragon is quite... weird. Firstly, it shows no bossbar, which is odd as with /summon, the wither will, it goes around in circles doing nothing, and has no death animation.
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True, but the dragon will only do the nbt data's command. You cannot spawn a dragon that acts as it would in the end without doing it via crystals or spawn eggs via mods like OreSpawn.

Except that that literally spawns an ender dragon that acts exactly like the boss fight dragon... The going in circles is it trying to find the exit portal, he'll stop spinning after a bit last I checked (which was in 1.19 snapshots).
The boss bar is bound to the whole fight (per mojang comment somewhere on mojira, not bothering to search), for the wither, the fight is the wither, for the dragon it is not.
Death animation works properly for me (with the correct dragon phase, at least).

@Dhranios it was Dinnerbone's comment on MC-83239.

Can confirm in 1.20.3 with command /summon ender_dragon ~ ~ ~. The dragon will behave normally if the command /summon ender_dragon ~ ~ ~ {DragonPhase:0} is used
Note: the dragon will gradually go into a wider and wider circle, however its Y height will not change
Resolving this as a duplicate of MC-82911.
Please report the circling issue, if you believe this to be a bug, in a new report with detailed reproduction steps and a video showcasing this in the latest release/snapshot
Using NBT
/summon ender_dragon ~ ~ ~ {DragonPhase:0}