Fall back IP
What i understanding regarding fall back IP is that if all virtual servers in pool associated with wide ip are down then DNS will resolve to fall back ip but in my scenario it's happening differently. I have included primary DC and DR virtual server/ip's in pool and fall back ip as Primary DC virtual server IP but when both virtual server are down then in nslookup it's showing both primary and DR VIP ip's instead of only primary DC IP in fall back IP.
You need to have the alternate set to fallback for this to use that IP entry.
Also, just to expand - I have just experienced this too and I understand that there is a level of caching done within DNS express in order to try to expedite the reply, you are also using a LDNS (presuming 10.10.10.10 isn't the DNS listener) to resolve this host you could get old answers. I disabled DNS express in my profile and it worked as I expected it should.
being new to the GTM I am also still learning, so this is based on my troubleshooting only.