My Suzuki SX4 had 3 settings - FWD, on-demand AWD, and locked-in AWD. BUT, over 35 mph it would kick out of locked-in until you put it back in. So, this kind of behavior is not unheard of.
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2017 LT 2.4L AWD purchased new July 2017
2001 S10 Crewcab 4.3 4wd
2006 GL1800 Premium Audio
1978 frame, '75 GL1000 project "The Hunley"