This is mentioned in the Steinberg knowledge base. When you select a MIDI track and record enable it, the track is no longer in ASIO-Guard mode and the sound mutes as it switches over to real-time mode (and your normal latency.)
This seems to be in addition to what Martin describes.
I can't see a specfic reference to that in the Cubase 8 issues - https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/cubase-8-version-history-and-issues-and-solutions.html . The sound neither mutes on other plugins or VE Pro under very favourible conditions.
I do have a feeling though that some of this might be Steinberg's end, you do tend to get a CPU spike when switching tracks in general (though that hasn't caused me any audio problems save for VE Pro). Though I can't pretend to understand the inner workings of these things, if asioguard's delay is 90ms, I guess a dream scenario is being able to seamlessly switch from prefetched audio to live latency audio somehow 90ms afer a track is enabled. Might not be possible though.