I experienced the same error code, 49187. Below is some background context from my situation and my resolution at the end.
I had re-imaged a MacBook Air as a dev machine and forgot that I wanted to partition the drive.
I, too, ensured file encryption and Time Machine were not active (and used
tmutil to thin snapshots) but still experienced the same partition error as the OP. I then took the scorched earth approach; I was going to re-format entirely. I first ensured I had a Mojave installer on a USB thumb drive (using DiskMaker X) then rebooted into recovery mode (Cmd-R).
There, I started Disk Utility to erase and partition. In fact, it failed there as well. After it failed, it showed used file space of roughly 1gig and 75gig of 'other' (which is actually the size of the partition I attempted to add).
The resolution, fortunately, was at hand. I was able to Unmount the APFS drive and actually erase/re-format it with APFS and then create the additional partition.
I am not exactly sure why this worked. When I looked at my drive initially in Disk Utility, it said that it was shared by 4 volumes although I hadn't partitioned anything myself. Once I unmounted and erased the entire drive in Recovery Mode, it showed as One Volume. I wonder if any recovery volume(s) added during High Sierra or Mojave's install prevented APFS partitioning to take effect.