Legit domains are being blocked in Pihole v5

They are not compatible in either V4 or V5, but importing them is handled differently in V5. They are not in HOSTS format. If you look at the output from a gravity rebuild (pihole -g), you will see the following:

 [i] Target: https://easylist.to/easylist/easylist.txt
  [✓] Status: Retrieval successful
  [i] Received 33321 domains, 22804 domains invalid!
      Sample of invalid domains:
      - 2.0]
      - (easylist.subscription@gmail.com).
      - &ad.vid=$~xmlhttprequest
      - &advertiserid=$domain=~bee.gl|~cpashka.ru
      - -ad.jpg?

Even if some of the domains are parsed, they are likely to be imported not as intended to be used in that list. As shown below

pihole -q google.com

 Match found in https://easylist.to/easylist/easylist.txt:
   transparencyreport.google.com 
   maps.google.com 
   google.com.au 
   google.com 
   google.com 
   google.com 
   google.com 
   google.com 
   mail.google.com 
   maps.google.com 
   maps.google.com 
   google.com 
   mail.google.com 
   google.com 
   accounts.google.com 
   www.google.com

There is no reliable way to parse these lists, and using them causes problems as you have seen.