Synchronizer chains not recognized

In my MAX10 project no synchronizer chains are recognized. As far as I understand a synchronizer chain begins with a asynchronous path to a register with is either:

• a signal from a process that is clocked by an unrelated clock (different clock group) or

• a toplevel signal that has not been associated with a clock by means of set_input_delay or set_output_delay

My single-bit sinchronizer looks as follows:

-- altera vhdl_input_version vhdl_2008
library ieee;
    use ieee.std_logic_1164.all;
    
entity bit_synchronizer is
    port(
        clk      : in     std_logic;            -- clock
        async_i  : in     std_logic;            -- async. input signal
        sync_o   : out std_logic            -- synchronized output signal
    );
end entity bit_synchronizer;
architecture rtl of bit_synchronizer is
    signal meta    : std_logic;
    signal sync : std_logic;
    
    attribute syn_preserve : boolean;
    attribute syn_keep: boolean;
    attribute syn_replicate : boolean;
    attribute syn_maxfan : integer;
    
    attribute syn_keep of meta : signal is true;
    attribute syn_keep of sync : signal is true;
    attribute syn_replicate of sync : signal is false;
    attribute syn_replicate of meta : signal is false;
    attribute syn_maxfan of meta : signal is 1;
    
    attribute altera_attribute    : string;
    attribute altera_attribute of meta : signal is "-name SYNCHRONIZER_IDENTIFICATION FORCED_IF_ASYNCHRONOUS";
    attribute altera_attribute of rtl : architecture is "-name SDC_STATEMENT ""set_false_path -to """;
begin
    process(clk)
    begin
        if(rising_edge(clk)) then
            meta <= async_i;
            sync <= meta;
        end if;
    end process;
    
    sync_o <= sync;
end architecture rtl;

I try to embed both the SDC constraints and assignments in the VHDL code to ease reuse. However, even when explicitly putting the same commands into the SDC and QSF file the synchronizer chain is not recognized. Neither the assignments nor constraints are listed as ignored in the reports.

Any ideas?