UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0

Steps to reproduce the problem:
1. Configure system to use proxy pac file
Proxy file looks more or less like this
if Ip/2 is even
then go proxy1
if IP/2 not even
then go to proxy2
if all proxies are down
go direct (bypass proxy)
2. Surf internet
3. Chrome marks both proxies as down and go direct, while company policy dont allow direct traffic to internet from users

What is the expected behavior?
for about 2-3 weeks there has been ongoing issue in company.
I got few k users across world and seems like many of them got issue ONLY chrome. It doesn't replicate to firefox, or IE.

What went wrong?
Problem:
1) Internet is sometimes not working for users. Refreshing page few times fixing problem. (display page to check proxy/firewall settings)
2) Chrome is marking proxies as "down". This is visible in somelink like
chrome://proxy

Did this work before? Yes about 2-3 weeks ago

Chrome version: <Copy from: 'about:version'>  Channel: stable
OS Version: (Windows 7
Flash Version: Shockwave Flash 29.0 r0

design of network
- http/https traffic should go via proxy servers. Proxies NAT traffic, so it's easy to find on firewalls if source IP belongs to proxy or to original IP of user and then its being forwarded to firewall
USER IP -> Proxy (NAT HERE) -> Firewall -> Internet

Findings so far
- only appears to chrome, no recored issue for IE, FF, safari etc
- i've found based on firewall logs that while issue is happening for users, logs on firewall are coming with source IP of specifiec user instead of proxy NAT IP. Which means that sometimes chrome is not using proxy settings.
its like USER -> Firewall (block due to obivious reassons, we only allow traffic from proxies, not from direct users)
- refresh or few refreshes fixes it, and page loads up, as it goes via proxy
- problems with proxies has been excluded as it happens across many company units across world wide. Also we got implemented redudancy for proxy settings and user still should be redirected to another proxy if there issue with proxy.

- root cause seems to be related with our filtering policy proxies. By default we are dropping traffic which is categorized as advertisments. When adding people to bypass list which orders proxies to not drop advertisments traffic  issue it NOT happening. I cannot consider to implemented this as pernament fix, as I'd need to add whole company to bypass list.
- clear cache seems to be fixing issue for few hours, or max 1 day