Thankfully I bookmarked a thread where a similar question was asked on the ESS / MSS subforums and Uber-Guru Siddharth anwered it:
Receipt currency field configuration for travel expense in ESS
I.e. entries in TCURC that don't have an exchange rate configured in TCURR are excluded from the list. I keep forgetting about this myself because we have exchange rates for all currencies in our system, thus this "filtering" doesn't noticably occur for us. 
Can you check whether this applies for your problem?
Cheers, Lukas