Fluids are placable on the sides of blocks that themselves are out of the player's reach.
Video showcasing the issue:
https://www.youtube.com/watch?v=FSnAjoAQUb0
Additional Information:
While producing the bug, I also found that the reach distance between creative mode and survival mode is slightly different. The blocks I was unable to reach while in survival mode were reachable in creative mode.
Environment
java version "1.8.0_202"
Java(TM) SE Runtime Environment (build 1.8.0_202-b08)
Java HotSpot(TM) 64-Bit Server VM (build 25.202-b08, mixed mode)
Windows 10 64-bit operating system, x64-based processor
Duplicates MC-170933 and MC-185442 (or MC-127223). The reach distance discrepency is intended in Creative Mode vs. Survival.