/data does not allow multiple target entities

Before entitydata @e {} was available, so this is definitely a bug which I can confirm.

Try this /execute as @e run data merge entity @s {}

That's not what this bug tracker is for, @unknown, sure, it's a workaround, but the goal here is to make issues clear to the devs, and get them fixed.

Confirmed for 17w47b.

Confirmed for 17w48a

Please use the /execute command if you want to use /data on multiple entities

But why? Is there a specific reason to not allow this if it works perfectly fine with {{execute}}?

It's an implementation detail. Right now it's the risk of breaking something with a bigger change vs. the fact that /execute is a way to achieve the wanted behavior.

Then it should be WF or better postponed.

Relating bugpost MC-121864 ("/execute store' does not allow multiple target entities") has also been resolved as "WaI", but with the comment of Mr. Adams saying that it is for now intentional, but it may be revisited in the future.

I agree that it may be better to use a different resolve reason for cases like this one, and say since a long time that it should be considered to add resolve reasons here on the bugtracker (or usage of "postponed" more often, if it fits); those resolve reasons were and still are occasionally the cause why some people argument with exactly these resolve reasons against (possible) changes and justify their upsetness against Mojang or their decisions.

If certain issues are planned to be addressed in the future for certain, it'd be at least a better way to communicate this to the public if such an according resolve reason would be chosen, even if it may take a while until it's re-addressed, or, in the worst case, the decision to really do so may change in favour of something else.