Hmm, well anything left on the ground, bed or counter, is open fur Savannah discretion. It's susceptible to be chewed, eaten, or peed on. It's just their way of training us.
Younglings are even more cantankerous. And likely to test ones endurance. My Cats have taught me to put litter boxes where they like to do their duty. Have 5 cats, 7 litter boxes, and 6 cat trees.
Would definitely schedule the Neuter asap. 5 months is a good age to have it done. And if the hormones are kicking in that early, it will curb that tendency, and rule out that being the problem.
Wood pellets litter,? While commonly used with some catteries, find it not the best litter of choice. May want to add a second box, and try a different litter.