No that doesn't sound normal. A regulator usually holds the voltage within 0.1 volts.

I don't know your system, but typically 12 volt systems will regulate around 14.0-14.5 volts.

Do you have any intermittent loads like flashing lights, strobes or wig-wag that draws high amps? That could be a cause.

Can you adjust your over voltage alarm threshold on the MGL? If so try increasing it to 16 volts or higher.

Does your electrical system have a big capacitor in the charging system? It could be a cause if it has gone bad.