The problem has been encountered on a Spigot 1.17.1 Minecraft Server.
Using version 5.0.2 of this plugin.
The command used is crates give <player> <crate> 1
The problem is that the number 1 is not respected and sometimes gives a stack of keys to the player.
The reason behind this weird behavior is under study and not found yet, so it might be categorized as a bug.
Thanks for contributing to the open source community, and for your time by reading this.

Looks like it bugs and gives stacks when the command is runned on first time on a player. Still under study.