Partition Created in Mojave Not Visible in Yosemite

I've got a number of expensive apps that run native on Yosemite (10.10.3). The rest of the time I'd like to run my system on whatever the latest MacOS is (Mojave at the present time). In the past I've done this by creating a partition on my HDD and then booting up in whichever OS is appropriate for the task at hand.

This time, when I boot into Mojave and open Preferences/Start Up Disk, I can see both OS versions and both partitions appear on the desktop. However, once I boot into 10.10.3, the Mojave partition is not visible in Start Up Disks, nor does it appear on the desktop. This makes it impossible to boot back into Mojave.

I assume that there's some issue with 'old' Yosemite recognizing the Mojave-created partition. However, since I had to use Option+Command R to create the partition, I'm stuck using whatever the latest version of Disk Utility is to create the partition.

Does anyone have a suggestion on how to workaround this?