PDA seeds element size problems

Question

I encountered an oddproblem with the seeds array when searching for a PDA via macro #account(init, seeds ...)].

Since each of the seeds array elements can be up to 32 bytes, I want to use the first 32 bytes of the text as a seeds element. I wrote a macro that converts a String to an array of bytes with the first n bytes.

macro_rules! to_trimmed_bytes {
    ($text:expr, $range:expr) => {{
        let bytes_slice: &[u8] = $text.as_bytes();
        let mut bytes_array: [u8;$range] = [0;$range];
        for i in 0..bytes_slice.len() {
            bytes_array[i] = bytes_slice[i];
        }
        &bytes_array
    }}
}

The accounts struct with the seeds looks like this:

#[derive(Accounts)]
#[instruction(text: String)] 
pub struct CreatePost<'info> {

    #[account(mut)]
    pub authority: Signer<'info>,

    #[account(init, seeds = [b"post", authority.key().as_ref(), to_trimmed_bytes!(text,32)], bump, payer = authority, space = 800)] 
    pub post: Account<'info, Post>,

    pub system_program: Program<'info, System>,
}

Now, when I try to run this, I am getting the following compilation error message:

&bytes_array
^^^^^^^^^^^^ expected an array with a fixed size of 4 elements, found one with 32 elements

After a little research, I discovered that these 4 elements are defined by the first seeds element b"post". It always allows the size of this first element, e.g. when the first element of seeds is of 7 characters (e.g. b"newpost") the error looks like this:

&bytes_array
^^^^^^^^^^^^ expected an array with a fixed size of 7 elements, found one with 32 elements

Why is this first seeds entry a limiting factor to other elements of the seeds array? Is there a way around this?