Redirect on HTTP::host does not work

I believe the issue is that you're not terminating evaluation after the first condition. You may be matching on the first condition, but the iRule is going to continue parsing the iRule. Try moving everything into a single if/elseif structure:

when HTTP_REQUEST { 
    if { [string tolower [HTTP::host]] starts_with "ourdomain.com" } { 
        HTTP::redirect https://www.ourdomain.com[HTTP::uri] 
    } elseif { [HTTP::uri] eq "/" } { 
        HTTP::redirect https://www.ourdomain.com/abc/portal/place
    } elseif { ( [HTTP::uri] contains "/mydomainLandingPage" ) } {
        HTTP::redirect "https://www.ourdomain.com/abc/portal/place/PageNotFound/"
    }
}

Also, you don't need the or ([HTTP::uri] eq "") section. An empty URI will always, at a minimum, contain a "/" character.