I work for Denver Public Library and we ran into the same issue when we tried to post Ebsco permalinks on a Drupal 8 site.
As a workaround we are able to pass the URLs through a URL shortener and post that shortened URL.
A colleague of mine also remembered that they used to have to change permalinks before posting back on D6. They demonstrated that if you change a link from
https://ezproxy.yourlibrary.org/login?url=https://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=100123456&scope=site
to:
https://search.ebscohost.com.ezproxy.yourlibrary.org/login.aspx?direct=true&db=aph&AN=100123456&scope=site
the second link would work with a D8 and above URL field while the first would encounter the encoding issue.
I have shared this information with Ebsco and am creating a ticket for ProQuest. However, if nothing changes with the way they create their permalinks the URL shortener workaround has been effective for us.