Strange behavior of the web-proxy

Hi all,
We have a RB1100Hx2 with RouterOS 5.26. Web traffic is redirected to the internal proxy server, which forwards the request to the squid. Until now, this scheme worked perfectly. It was recently observed that some requests are transmitted to squid is not quite correct . When using tcpdump found that mikrotik periodically transmits instead of the hostname ip address. Examples of headers for one the same query:
Invalid request

GET http://2.19.118.235/library/capi/wt_capi.js HTTP/1.1
Host: i.s-microsoft.com
Cache-Control: max-age = 0
Accept: * / *
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.152 Safari/537.36
Referer: http://www.microsoft.com/ru-ru/default.aspx
Accept-Encoding: gzip, deflate, sdch
Accept-Language: ru-RU, ru; q = 0.8, en-US; q = 0.6, en; q = 0.4
If-None-Match: "463c32c671f7ce1: 0"
If-Modified-Since: Thu, 12 Dec 2013 19:38:46 GMT
X-Proxy-ID: 1643700960
X-Forwarded-For: 192.168.0.80
Via: 1.1 192.168.0.252 (Mikrotik HttpProxy)

Valid request

GET http://is-microsoft.com/library/capi/wt_capi.js HTTP/1.1
Host: i.s-microsoft.com
Cache-Control: max-age = 0
Accept: * / *
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.152 Safari/537.36
Referer: http://www.microsoft.com/ru-ru/default.aspx
Accept-Encoding: gzip, deflate, sdch
Accept-Language: ru-RU, ru; q = 0.8, en-US; q = 0.6, en; q = 0.4
X-Proxy-ID: 1643700960
X-Forwarded-For: 192.168.0.80
Via: 1.1 192.168.0.252 (Mikrotik HttpProxy)

Has anyone encountered such behavior of the web-proxy?