The firework limit is 25 at once but is a server-wide limit. So if other players are also lighting fireworks that would count towards the overall limit.
The underlying issue is still unknown. I'm thinking the collection that stores active fireworks is not pruning expired/exhausted fireworks causing the limit to always exceed 25. I'll have to do some internal testing to determine if this is the cause.
If this were to be the issue I can modify the server files and fix it until Facepunch can. Otherwise, there's nothing I can do. I will keep this report closed unless I find a solution, in which then I'd mark as fixed.
I will look into this further this weekend.