http being changed to https through url shortening

We have an odd issue with tweetily on our website. Randomly, some tweets posted with the shortened url are changing so that when you click them in twitter it is taking you to a link starting with https instead of http. This is happening between the url shortened by tweetily and the t.co shortened url from twitter. So the problem could be with twitter. It seems the already shortened url is getting shortened again (or lengthened) by twitter and there is no way around this. But the final full url has the wrong protocol attached to it. The really odd thing is that it is not happening to every tweet from tweetily.
Anyone have any ideas?

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