Concur w/above, probably just some air was in the cooling system. At this point I’d be more concerned about the sludge in the bottom of the plastic overflow reservoir than the drop in coolant level. If you are certain that bottle was clean at the time of the engine replacement, suggest to do a couple of drain and refills of the entire cooling system and sieve what comes out to see if there’s further problems lurking inside the engine.
If that all turns out ok, it’s always a good idea when refilling the cooling system, afterwards to open the radiator cap (not just the plastic bottle cap) and idle the engine in the driveway for 20 minutes until the radiator cooling fan comes on a couple times. Make sure while doing this the heater is set to “max heat” so you get maximum coolant going through the heater core. At some point you should notice a big surge of flow into the radiator which confirms the thermostat is opening. That process usually rids the cooling system of all the entrapped air. Especially for trucks which sit higher off the ground so the radiator cap is the highest point in the system. You can ensure this by driving the front wheels of the truck on ramps of course.
If all that still doesn’t solve the loss of coolant, either it is going into the combustion chambers due to a faulty head gasket, it is leaking into the oil again due to a faulty head gasket or oil cooler, it is leaking into the transmission fluid (if your truck has an automatic), or you have a small external leak somewhere (hose connection, radiator, etc) that is evaporating before any drops of coolant are finding their way to the floor of your garage. For the head gasket possibilities, mechanics have ways to test for that and do it all the time; for the latter there are dyes that can be put in the coolant, then you use a special lamp to spot where the leak is occurring.
I should add I own a Corolla of similar vintage and been losing a little coolant. About a cup every 6 months. I just recently discovered it’s coming from a leak where the metal return hose connects to the water pump housing. My solution is to just top off the coolant every 6 months.