A thought occured to me that Hikari's experiments could not possibly be the cause for Rakka's hair problem. The last haibane to be born before Rakka was Kuu. It was no less than one year ago, possibly more (see Kuu's flashbacks). If Hikari used the carrier at the time, the donuts would've been known. So, she used it during the christening of Rakka. But, since halo occupied the carrier, she could only do it after halo was delivered. Therefore, the contamination could not be the reason for whatever hair issues. I hope nobody suggests that Hikari held a hot halo aloft while playing with the carrier.

FRIDGE: Kuu was not necesserily the last one, actually. It could've been one of the kids. Still, Reki's surprise shows that a cocoon has not appeared for a while; enough for donuts to become known.

Steven helps me out:

You're making a fundamental mistake. You're assuming that when Hikari received the mold from the Renmei, that the halo was already in it.

She was given the mold plus a package of ingredients. She baked the halo the morning it was given to Rakka, likely just before they all entered and greeted Rakka.

When the Renmei gave Hikari the mold, it was empty. She used it to make doughnuts before she used it to make the halo -- which is why the halo wasn't quite normal.

Thanks. And for some reason picturing all this makes me laugh again.