Our users have what I thought was a fairly simple request when working with Lightning for Outlook, they want to relate an email message to both a Lead and a Campaign. Since the Activity/Task record can relate to both a WhoId and WhatId, I figured this wouldn't be a problem.  However, I've witnessed and recorded a user experience the following behavior in the Add-in and I cannot figure out what is causing it.

https://www.screencast.com/t/BdiMY0ddBQI

As you can see, each time she tries to associate the message with the lead it disconnects the campaign.  (This does not seem to be the case for Contacts but both Leads and Contacts can be associated with SF Campaigns.)  And each time she clicks the campaign it disconnects it from the lead.  I also had her try to remove herself from the association and just try the lead and campaign but the experience was the same.  Shouldn't an activity be able to be associated with a WhoID (the Lead) and a WhatID (the Campaign) at the same time?  FWIW, we're using the Modern Email object for storing email messages.

Secondly, while this may be related, they also see the following error on occasion.

"You can't send an email to a lead when there's a value in the Related To field. Remove the lead or the Related To entry."

https://www.screencast.com/t/BrkkaWWGapfu

I've tried searching for this error (both in a couple of search browsers and in this Community) but only found these two articles but neither has responses as to what to do about it.

https://success.salesforce.com/answers?id=9063A000000pLGaQAM

https://developer.salesforce.com/forums/?id=9062I000000IMQoQAO

Thanks in advance for any clarification or help you can provide on these things.