UC Browser belongs to a class of browsers called "proxy browsers". That is, you are not directly visiting IRI. UC Servers themselves visit IRI, load the page, compress and process the page (and can do other stuff). Thereafter, you are getting the page from UC Servers, not from IRI. Then of course, there is the Wifi and other intermediaries. So, there are lots of places to inject ads.
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It is certainly strange that you are seeing these popups only in IRI. Anyhow, you have done your diligence. We shall look into this...
more... further.
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Thanks for your help.