It is not normal.
IMHO the reason is loss of excitation which may caused by:
- over voltage (greater than 15V) at the alternator output
- loss of output voltage (e.g: due too short requlator brushes, starting to float on higher RPMs,
thus interrupts the excitation: the rotor is not getting supply)
Both can be tested by a voltmeter connected to the battery terminals.
For the second test it could be useful to turn on some electric consumers (high beams, blower @ full speed),
because the voltmeter will show a more significant drop in the readings as the charging stops.