Easy, just if it's a neutral unit. Monster's faction ability is to keep the last monster or neutral unit played.
You can play around that. If you already have a strong monster/neutral unit on the battlefield that you want to keep but you still need a few more points, steal a faction unit. If your last unit is wounded or just weak (arachas, etc) steal a stronger neutral unit if there's one in your opponent's graveyard. (Vezemir, 10str Regis)
If it wasn't a neutral/monster unit, that's a bug and you should report that to CDPR.