I believe the limit is based on the encrypted length, not the original length of the text. So yeah - you must have just crossed that fine boundary where it was just exceeded.
I agree that there are many valid use cases where this needs to be improved. Thankfully, it looks like this one is in the queue for revision at some point - hopefully sooner than later!
In the meantime, one workaround (albeit not entirely elegant) is to save the long note as a text file and attach it to the vault item. It will stay secure and with your item, but the two downsides are that you have to download it every time that you want to view it and a Bitwarden export will not save the file attachment (those have to be done manually).