I assume you have [speaker icon]>Sounds>Recording>[UMC1820]>Properties>Advanced also set to 48000.
Check out this thread: Problems Using Behringer UMC1820 interface
and this one: Behringer UMC1820 PC Issue
For multi-track recording with Behringer, you may need to use ASIO. First, have you tried MME or Windows DirectSound? You can also check for a different driver on the Behringer web site. If you need ASIO, other software packages including Reaper and Cakewalk may support multi-track recording from Behringer with ASIO. For licensing reasons, Audacity does not ship with ASIO.
If you have the drive, you can compile your own version of Audacity with ASIO, which should allow multi-track recording with your UMC1820. See here: ASIO support in Audacity 2.4.2 - #13 by jademan (BTW this is what I do for my multi-track Behringer).
I hope this helps. 