The best practice is to clear cache and cookies when changing IP (remember to clear flash cache too). Alternatively you can use incognito mode.
Furthermore I have strong feeling this has to do with VPN servers being used by HMA during the connection. On HMA support page, they actually have acknowledged that they're testing new servers and often times these may not reflect true location (I recall experiencing these same issue with Psiphon VPN showing a mismatch between selected and apparent locations):
During the recent upgrades of our network we have replaced certain
servers with the new, better ones, allowing greater speed and a more
stable connection. We are aware that when connected to these new
servers, our customers are getting Google results in Czech. As these
servers are brand new, it may take some time for the Google to reflect
their true location.
Additionally I suspect, the VPN configuration somehow gets buggy especially when congested with traffic such that it "auto-picks" servers close to your actual location, probably because there is faster speed and lower latency than servers that are much further from real location.
You may therefore wish to check if the IP location of the chosen connection is correct on geoip.hidemyass.com or similar IP checkers (such as maxmind.com or whoer.net).