Well, A few thoughts.
I know, you probably already did, but check the trans fluid level on flat ground at initial startup.
The most accurate method is to shift through all gear positions at startup, wait about 60 seconds and then check the level.
To get the most accurate reading, pull the dipstick BEFORE starting the engine and running through the gears.
Then check the level.
This removes the potential of fluid splash within the trans causing a false read.
Remember, the fluid will read at the full cold mark on the grid. Adding to full when cold will result in overfill once fluid is warm. Trans fluid has a HUGE expansion rate as it heats.
If the level is correct, then the torque converter is likely draining down overnight.
If, by leaving it in reverse, the gear engages within 5 seconds, you are seeing the converter refill itself before it can pressure up the trans.
This anomoly will happen to most vehicles if allowed to sit unused for a few days.
If the gear engages within 5 seconds, and the trans is full, it likely is the nature of the beast and will cause no harm.
If you must throttle it, with the fluid level correct, to get engagement, the pump circuit has a leak and likely will fail.