Should add that I can only repro this bug when setting the display size to 90%. Autohide works fine on the other scaling options.

I can replicate this.

Repro steps:

set shelf to "autohide"
Start with regular zoom (ctrl+shift 0)
put cursor at bottom of screen. Notice that shelf appears
zoom out one step (ctrl-shift "-")
put cursor at bottom

Expected:
shelf appears

Actual:
shelf does not appear, or appears only occasionally.

--

when the shelf actually does appear, it seems to have a slight border around it -- this may be a clue to the problem.

If I put the shelf on the right or left side, then I do not experience this issue. 

I am fairly certain that the issue has to do with the position of the mouse. If I move the the mouse very slowly to the bottom of the screen, it triggers the shelf when a few pixels of mouse are still visible. OTOH, if I move the mouse very quickly to the bottom of the screen, then the shelf does not show and only a single pixel width of mouse is visible. 

I hope that helps!

Can you share your chrome version?

In my case, it was an older Chrome OS. I do not remember exactly what version, but I believe 68.

I have since updated and can no longer replicate with the steps I gave above.

Current version: Version 71.0.3558.0 (Official Build) dev (64-bit)