Like I mentioned earlier, the physically closest data center doesn't necessarily mean it's the fastest.
For sake of argument, let's say there's one data center in Germany, but for whatever reason, that data center is peered/backhauling traffic through Spain. Maybe a user in Germany is routed through a data center in Belgium because a route to the data center in Germany might go through France, then Spain and then back to Germany in this case, where Belgium might be a shorter hop back to Germany.
Take another more local example... you are physically standing next to the data center on your cell phone. You are so close, you can literally reach out and touch the building. However, a network connection there from your cell phone goes through a cell tower, then backhauled to the cell company's regional POP, then that is routed to NOC, which might be peered to an unrelated network provider, which then sends the traffic to a different NOC that is peered with the physical data center you are standing next to. Physical proximity doesn't mean your network traffic somehow magically gets straight it (in my example maybe your network traffic had to take a 1500 km physical route to get into the building you can touch).
That's just some examples of what can (and often) happens with network routing. In an ideal world, every data center and NOC are interconnected with every other data center/NOC in the world, but that's also an impossibility. The best you can do as a data center is constantly improve your interconnections/peering (usually focusing on the worst routes).
Also, Google has a physical data center in Las Vegas as well, but when I'm physically in Las Vegas I connect to Google's Los Angeles data center... again... same thing. The network route is faster to go to Los Angeles in that case than it is to go to the Vegas data center for whatever reason (peering in Las Vegas probably isn't as great as it is in Los Angeles... same reason I hit a Cloudflare data center in Los Angeles). It has nothing to do with them forcing a bad route to you because of your plan.
Back to your original question though, you can test basic data center routing of a Free vs. Enterprise domain, and I've never (ever) seen a case where they are routed to different data centers from the same device at the same time based on domain. The way Anycast works is the network equipment (before anything even gets to Cloudflare) is routing the user's traffic to the fastest data center for them.
If you are really that worried about it and you think Cloudflare is giving your users bad network routes, it's easy enough to turn off the Cloudflare proxy and see if your site gets faster for users somehow.