In order for bash completions to work, they must be installed in the respective folder. Often that folder is writable only by root.
In order words, using a command line the following now is successful, providing indicative warning message.
meson builddir --prefix
meson.build:140: WARNING: User provided prefix '.../inst' differs from bash-completion one '/usr'. Disabling completion.
/cc @pq can you confirm this fixes the issue on your end? Fwiw other meson projects (systemd) suffer from this problem.