I've noticed this behavior for a few years now. If an Event is created from CRM and sent to participants and the participant accepts the invitation using Outlook client, Zoho event won't be updated as "Going" it only works if the recipient accepts it using a web browser, like Gmail our Outlook on the web.  

Since Outlook Client is still a dominant client for many people (at least in my industry) this makes the Participant tracking unreliable, therefore my users don't want to use it. 

i reported this a couple years ago via chat support, just checked today and still doesn't work :( 

it would be a lot cooler if it did ....