Is it a sinful act to seek the shelter of some other Guru after shunning the first one?

No, it is not so. If you find that your Guru is not
a Sant Satguru, Shun him and go in search of some perfect
Guru. There is nothing wrong even if you have to shun
more of such incomplete and imperfect Gurus.

Jabb Takk Guru Miley Nehin Sancha,  
Tabb Takk Guru Karo Dus Pancha  

You can seek the shelter of more than one Guru by the time you succeed in finding some perfect Guru.
However, it is sin to speak against the previous Gurus. In fact, finding a true Satguru for seeking liberation of the soul is really a great noble job.

Ja Guru Te Bhram Na Mite, Bhranti Na Jiv Ka Jaye.  
So Guru Jhootha Janiye, Tyagat Der Na Laye.  

A Guru who cannot remove the illusion and delusion, is a false Satguru, leave him.