.ml tends to be one of the most abused top level domains for malware, spam, etc (in terms of ratio of malicious to non malicious domains) similar to .top, .buzz, .club, etc. So, many DNS filters on company networks simply filter all domains of these TLDs (and maybe whitelist a few known good ones) since they tend to be almost certainly malicious.

I filter them on my home network too via pihole (though not .ml)