We wanted to use your plugin, but we have this bug now .. it is being spammed en masse in the logs, is it possible .. to fix this bug?
[Warning] Calling hook OnMaxStackable resulted in a conflict between the following plugins: ChestStacks - 12 (Int32), Flashbang (1 (Int32))

That happens because that plugin is trying to change the stack size of the flashbang as well. Whether that be a smoke grenade, it still affects the stack size.

Is it possible to fix somehow?

I'm not sure how that plugin works. I'm not sure if that is because I return null, it conflicts with it? I'm not sure. @Wulf do you have an idea?

Hook conflicts occur when more than one plugin return different non-null values. To avoid a hook conflict, one of those plugins has to return null or the same value as the other. There are several ways to accomplish this.

A) One plugin will have to be aware of the other, to call its hook to see what value it wants to return, OR one plugin exposes a unique hook, allowing another plugin to override the value it should return
B) One plugin will have to generically re-call the hook to allow any other plugin to override it (doesn't require any plugin to know about any other specific plugin, but had scalability limits with how many plugins do this).

The way I see it, generic stack size plugins such as Stack Size Controller or Stack Modifier, should allow any other plugin to override. Slightly generic plugins such as Chest Stacks should also allow overriding, so that really specific plugins such as this one can determine the stack size.

There may be yet another option for this particular case. Assuming this plugin uses a unique skin id, stack size plugins could theoretically be designed to allow this plugin to register a stack size with that plugin, for that particular skin id, so that no special hook negotiation is needed.