Leyline of Singularity Overflow behavior [Investigating]

I just pulled a Leyline of Singularity as the sixed card triggering the overflow ability.

My opponent has Urza on the battlefield with +4/+4 from Urza's factory.

While the dialog for chosing "not now" or "confirm" is active, urza is a 4/7 instead of the 8/11.

After confirmation he is a 8/11 again due to end of turn.

How "remove all buffs" is intended to behave in this scenario?

Unlike buffs applied by casts, buffs given by supports or vanguards are only removed until end of turn.

Intended?