Android's ecosystem supports two types of partition layout: the traditional partitioning scheme (where the ramdisk is present in the /boot partition and mounted as rootfs and system is mounted at /system), and the newer A/B partitioning scheme, where the system is mounted as rootfs. Google has made system-as-root mandatory for devices launching with Android 9 Pie (as part of the changes done for Project Treble), but A/B partitioning continues to remain a recommended but optional feature due to the changes it requires. This means that there are new devices that will launch with Android Pie, but come with a single "A-only" partition. For these devices, the OEM has to still ensure that the phone uses system-as-root, which in turn means that the ramdisk has to be merged into the system image in order to be mounted as rootfs. System-as-root is not mandatory for devices that upgrade to Android 9 Pie.
This scenario created an issue for users wanting to run Magisk on their devices. Magisk already supported system-as-root devices, but this was restricted to devices with the newer A/B partitioning scheme. Several Chinese devices, like the Xiaomi Mi 8 SE and a few others, and apparently the new Samsung Galaxy S10 too, that utilize the system-as-root setup without using the A/B partition would end up being unable to boot if Magisk was installed onto their Android Pie builds.
Magisk has now added in support for A-only system-as-root devices. This change is currently available in the Canary build channel with build number v18.2-e72c6685 (18111). You can also install the Canary builds through Magisk Manager by switching over the update channel to custom and pasting this link:
https://raw.githubusercontent.com/topjohnwu/magisk_files/master/canary_builds/release.json
Do note that the canary release channel is considered the "bleeding edge" of Magisk releases, and as such, is recommended for developers only and not for average users. Please do not install canary builds only for the sake of being on the absolute latest release.