Issue seems to be intermittent. I tested the same thing again and things are working fine. I'll get back if I able to reproduce consistently.

Any way as per screenshot, I saw the issue at least once. Is there any A/B experiment for this feature. Could you guys please check it on your side and confirm things are fine?

It is reproducible again consistently. I actually enabled flash for apps.facebook.com when flash allow popup is shown. Then I went back to chrome://settings, modified flash settings to remove exception for apps.facebook.com.

When I reloaded the farmville2 game (apps.facebook.com/farmville-two), I got a empty div where flash should load.

The same is reproducible in following games also. 

1) Candy crush saga - https://apps.facebook.com/candycrush/?fb_source=fbpage ("Get flash player button is shown. Upon clicking white screen is shown)
2) Flash pool - https://apps.facebook.com/flash-pool/ (Only white screen is shown)

However there are other games like "candycrushsodasaga" where things worked as expected. It seems to be an edge case, but a problem with chrome for sure.

In the case of Zynga, there doesn't appear to be any detection logic for the presence/ absence of Flash Player.  If a user goes and disables Flash Player via chrome://plugins, the experience is identical.

The recommended fix for this, would be for Zynga to either test for the presence of Flash Player and show a standard Download Flash Player link, or to directly embed a Flash Object so that a plugin placeholder image could be rendered.

In either case, since this behavior is present when Flash Player is disabled (i.e. it's not a problem w/ the detection logic of HTML5 by Default), this is something that will need to be addressed on their end.

Is there any recommended way to detect flash installation? Currently we use navigator.plugins to detect.

If above array contains an element with 'Shockwave Flash', we assume flash is installed. If not we take it as flash NOT installed.

When we simulate chrome HBD feature by modifying site engagement score to less than 30, sometimes navigator.plugins contains 'Shockwave Flash' element and sometimes it didn't contain the same element. 

In first case where navigator.plugins contained 'Shockwave Flash', swf is embeded, flash object is also loaded from zynga side and chrome showed a grey clickable screen to enable flash.

In second case where navigator.plugins didn't contain 'Shockwave Flash' and empty div is shown.

What we want to know why there is a difference in navigator.plugins when site engagement goes low. I assume chrome is altering that property and would like to know more on what basis it is done. I also suppose a bug on chrome side while altering this property based on site engagement.

Created a separate ticket for navigator.plugins issue with chrome HBD feature - https://bugs.chromium.org/p/chromium/issues/detail?id=665381

Able to reproduce this issue on Windows 10, Ubuntu 14.04 and mac 10.12.1 with chrome beta 55.0.2883.44 with above mentioned settings i.e by enabling the flag "Prefer HTML over Flash".

Note:
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If "Prefer HTML over Flash" flag is set to default, then we observed flash running is fine.

Since "Prefer HTML over Flash" flag is introduced from M55-55.0.2883.0, considering this issue as non-regression and marking it as untriaged.