watch the video, the unit does reinforce. You just have to move it again. I take those units from 1 or 2 back to 4.
Specifically, watch the last 30 seconds of the video. I take the unit with the reinforce build queued up, then move him over to the same spot at another bunker which finishes the reinforce
I noticed it while playing in a 4v4. I was able to reproduce the bug in a custom game with a bit less action going on.
It appears that German units are able to reinforce while standing in certain locations very close around any bunker. I was able to reproduce the bug using a standard non-upgraded bunker, as well as the MG bunker. At no time were my units near a building that provided reinforcements.
I am uncertain if this is a known bug already. I did not see it on the beta patch notes today as being fixed. Below you can find a video of me reproducing the bug. If this isn't a known issue, I can't imagine I'm the first person to notice it / abuse it during a game.. I've been exclusively playing Soviets for months up until this week as it was either switch sides or uninstall the game.
Using this bug, you could avoid ever needing to make a command bunker, thus saving resources, or build time.