TD or Not?
 

Punting situation.

A1 tries to catch the punt near his own goal line but drops it. B1 picks it up in the end zone.

The officials brought the ball out to the 20, first and ten for Team A.

Could I please have an explanation?

EDIT: NFHS rule set.

Since R never had possession and just muffed it, the kick never ended. As soon as the kick crossed the goal line it was dead and became a touchback. The kick does not end until R gains possession or if it becomes dead by rule.

A kick is a kick is a kick. It remains as such until possessed.

So if B1 had recovered the ball on the one yard line, would Team B keep possession?

I'm not a football official, saw this last night at my high school's game,

If the receivers (R) are the first to touch the kick after it crosses the LOS, it will be a first down for the team in possession at the end of the down, unless a penalty is accepted.

So K's punt is muffed by R88 at the R1 and the ball then breaks the plane - touchback.

K's punt is muffed by R88 at the R1 and possessed by K1 at the R1 who runs into the end zone, first down for K at the R1, because K cannot advance a kick.

K's punt is caught by R88 at the R1 and then fumbled, K1 can recover, and advance and when if the loose ball crosses the goal line it remains live.