Hey, this is because we have a 7-day edit timer across devforum posts, with exceptions only for the first post in certain categories.
While it would be nice to address this case since bulletin boards seem okay to edit for a permanent amount of time, this is fairly low impact and has a clear workaround (just use a single post, the first post, instead of having replies) so we recommend pursuing via the workaround.
Please file a feature request in Forum Help > Forum Features if you think this impacts your workflow significantly.