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I don't think the iframe is doing it. I would try to remove everything from the portlet, just write "Hello World" or something to see that it is there. If the issue goes away, the cause was somewhere in the removed code. Then try to add stuff, till you hit the issue again.

If the issue doesn't go away, it has to be somewhere else in your project. In really mysterious cases I usually create a new project and start adding till it stops working.

I had tried removing <iframe> from the portlet and it had resolved the issue. Even I deployed my portlet without <iframe> and in browser devtools added an <iframe> and the problem appeared again. Building a new project is a good idea. I will try building an MVC portlet instead of the Struts portlet (my current portlet is a Struts portlet) and check the result.

I must have written

<iframe> ... </iframe>

but I had written

<iframe ... />

The problem is a Liferay issue with <iframe>: https://issues.liferay.com/browse/LPS-7100
The issue has been closed but I found this problem existing in Liferay 6.2.

This actually isn't a Liferay problem. Iframes are not selfclosing. So you absolutely must have a closing tag.