You have a string attribute on a subobject and defined the maximum size to 256. However, when you try to set up the auto-numbering scheme it will only allow 80 characters.
This is working as designed and is considered a product limitation. The max length when auto-numbering is 80.
For enhancements/ideas, please join the Product Innovation call.
https://broadcomcms-software.wolkenservicedesk.com/external/article?articleNumber=18799