The warning for this is new in 1.5.24, but the behavior has been the same for 9 years. A leading '^' in a folder-hook or mbox-hook expands to "current mailbox", not "beginning of line".
See https://dev.mutt.org/doc/manual.html#mailbox-hook and https://dev.mutt.org/trac/ticket/3788 If you need beginning on line, the best way is to use parenthesis: folder-hook (^buildd$) source .muttrc.buildd
