When creating a new group notice, the Subject input accepts up to 64 characters, and the Message input box accepts up to 512 characters. These are faithfully relayed in their entirety when the notice is sent.
When viewing past group notices using the group inspector, the title's length is counted toward the length of the total message and must be under 512 bytes to be shown in its entirety. A max-length Subject and max-length Message will be 576 bytes in length, truncating the Message to only the first 448 bytes. (I think there may be a linebreak added in there too, eating up even more of the maximum length.)
As someone who regularly writes fairly lengthy group notices, this is incredibly annoying and has bugged me for years.
I'm guessing based on the nature of this bug that this is a database field that is too small rather than merely a UI problem. It might also be a protocol problem with how the group notices are sent when the viewer queries them.
If fixing the backend is infeasible, I would prefer to have the input box shortened to 447 bytes, to prevent problematic user input.
(I'm sure someone will come along soon proposing that Premium subscribers should be able to send longer messages. That elitist brainrot should be a separate feature request. This is a real bug that has been around since the dawn of time and seems like it should be easy to fix in several different ways.)