A AGM battery nearing end of life will act as you described. There is
not enough power for the start initially. The sealed AGM has the
property called "bounce back" that is essentially the residual chemistry
in the battery "recharging" so at some point in time it has enough
power to start the bike. It usually happens when stopping to get gas.
The bike is usually pushed to the side of the station and cussed and kicked
for about 30 minutes and then like magic it starts.
Some will say no that is a ring antenna failure. Could be, could be.
Some will say no your alternator regulator is headed south. Maybe, maybe.
With a digital voltmeter and a little knowledge you should be able to sort it out.