Strange. I have only it for sim2.
This is because xiaomi.eu is based on Chinese rom. It's probably matter of configuration which slot should work with 5G. Anyway buying Chinese sim tray is worth it. Dual SIM works perfectly fine with global version of this phone.
5G works but you need dual sim tray (5$ at aliexpress) because it's only enabled for SIM2
this rom is based on chinese rom and chinese version of this phone have dual sim tray by default and 5g also works there only with SIM2