I have a Drupal 7.79 and a Joomla 3.9.25 site on the same server, both running Civi 5.35.1 and both with loss of unsubscribe, opt-out and forwarding capability.
Flexmailer is enabled on both sites.
In the CiviErrors extension output (which I don't understand) I see the lines below (note the occurence of &).
[entryURL] => https://SITE-ONE/index.php?q=civicrm/mailing/unsubscribe&reset=1&jid=9099&qid=1380011&h=7d6e80ef654067bb
Full report at https://pastebin.com/vywyNWXH
[entryURL] => https://www.SITE-TWO/index.php?option=com_civicrm\&task=civicrm/mailing/optout&h=9ee5ca3feec72a53&jid=4304&option=com_civicrm&qid=557257&reset=1
Full report at https://pastebin.com/ZhByJSB1
I have had problems with munged Civimail URL's before & wonder if this could be a similar issue.
The emails sent and received do not contain munged URL's and the 'access CiviMail subscribe/unsubscribe pages' permissions appear to be set correctly.