I experience this all the time. I got maybe 88 sticks and then I got 14 sticks. They don’t stack. They just consolidate the one stack to 100 and then leaves another stack with 2 in. Trying to stack the two just switches their place in the inventory. Then suddenly, after trying for several minutes, dividing the stacks into many smaller stacks, combining them again etc, then suddenly make the one multi stack. I do not know why it sometimes wont stacks and other times it will; haven’t been able to find a pattern.
Idk what is going on, but this problem is still a problem and it is really annoying. If you play this game once in a while I cannot imagine you not seeing this?