Steps to reproduce:

We assume that ConfigFrequency of active proxies is relatively low. In real example it's 120 seconds. And CacheUpdateFrequency on server side is also 120 seconds.
In more simple words - it's highly possible that proxies will refresh configuration, while server daemon did not spot config changes in frontend.

Steps to reproduce:
1. Ha HostA monitored by active Proxy1, icon is green
2. Switch the host to monitor by active Proxy2 (host still is available in server db)
3. before server updated config cache, the Proxy2 has updated config cache
4. an item is scheduled and polled by the Proxy2 quickly enough, host detected as available and icon became green in frontend (but for short period)
4. a bit later server updated its cache and ... resets the icon to gray
DONE

Here are logs:

Proxy2 cache reload:

 23881:20180423:190828.371 forced reloading of the configuration cache
 23881:20180423:190828.377 received configuration data from server at "127.0.0.1", datalen 3453
 23902:20180423:190857.043 enabling SNMP agent checks on host "HostA": host became available

Server cache reload:

  6103:20180423:190828.377 sending configuration data to proxy "Proxy2" at "127.0.0.1", datalen 3453
  6096:20180423:190857.560 enabling SNMP agent checks on host "HostA": host became available
  6080:20180423:190907.954 forced reloading of the configuration cache

HostA availability status on server and proxies databases, different time poinds: before and after some changes. With comments:

19:08:21
			<<< HostA "monitored by" changed: Proxy1 => Proxy2
19:08:28
Server: 1
Proxy1: 1
Proxy2: 
			<<< Proxy2 cache reload:
19:08:29
Server: 1
Proxy1: 1
Proxy2: 0
			<<< server spot/got something and reseted host to unknown
19:08:30
Server: 0
Proxy1: 1
Proxy2: 0

19:08:56
Server: 0
Proxy1: 1
Proxy2: 0
			<<< an item polled and HostA detected as available
19:08:57
Server: 0
Proxy1: 1
Proxy2: 1
			<<< server received the availability update
19:08:58
Server: 1
Proxy1: 1
Proxy2: 1

19:09:07
Server: 1
Proxy1: 1
Proxy2: 1
			<<< Server config cache reload and icon magically reseted to unknown
19:09:08
Server: 0
Proxy1: 1
Proxy2: 1

Any further config cache updates will not update the icon.

The Proxy2 restart helps to update the availability icon on frontend side.

Note - we have such doc: https://www.zabbix.com/documentation/3.0/manual/introduction/whatsnew300#host_availability_improvements

It might be related to recent changes in -ZBX-12971-