[jepler]

bsd/vfs/vfs_vnops.c:

        if (write_offset == INT64_MAX) {
                /* writes are not possible */
                error = EFBIG;
                goto error_out;
        }

[skissane]

Thanks, that is indeed the answer.

I posted that as an answer to the question, acknowledging you as the person who had pointed out the answer to me: https://stackoverflow.com/a/78934138/2147204