What version of AdGuard are you using? What filter lists are you using? What OS are you using? Any antivirus/antimalware software?
One thing you might try is typing chrome://flags into the address bar, scroll down and look for the option for Experimental QUIC protocol and set that to disabled and restart Chrome and see if the ads appear again.
In that case, it's probably worth reporting here via the new reporting tool (if you click on the AdGuard Assistant icon on YouTube and select Report the webpage it'll auto-fill the majority of the needed information).