The average healthy adult male in a temperate climate should consume about a half gallon of water on top of a full day of Soylent, with wide variance depending on your specific needs. If you consume most of this during the day, then it’s not unusual to wake up a little dehydrated, though there shouldn’t be anything in particular in Soylent that would cause significant water loss so if it’s really extreme then it could be worth looking at other causes.
There have been other threads about this and in most cases people just aren’t drinking enough water due to liquid food removing the typical desire to have a drink with a meal. It could be possible that something else is going on though, not the least of which could be additional water lost in digestion if you’re experiencing intestinal irritation.
EDIT: Also if your other 50% of meals doesn’t contain much water then you’re missing out on a lot of liquid that you would be consuming if you were eating differently (or consuming 100% Soylent), so a gallon of water in that context seems pretty reasonable actually, if a little higher than average.