> This is a known problem and registrars are handling it

Nitpick: registries are handling it. Registrars usually just use the IDN tables provided by the registries. Mixed script rules in particular are a nightmare to implement perfectly when the tables are not given directly by the registry (for some ccTLDs mainly).

For gTLDs these tables are directly available on iana: https://www.iana.org/domains/idn-tables