I’m running Discourse 3.0.3, but this also happens on Meta.
I’m not sure when this happened, but it didn’t used to have a limit. We have a long template that we request people fill out for one of our groups. We’ve used it for years. But now when requesting membership, people cannot make any changes to the text in the text box, except to delete text. Once they get below 274 characters, they can add text again.
It’s easy to verify this without a long template, though. Just request membership in a group and try to type more than 274 characters in the membership request field.
On the admin side, I can still save long templates, and on the user side, the whole text still shows up in the text box. But it cannot be used.
Also, there is no warning to the admin or explanation to either the user of what is going on.