These 15 shares of IHS RSU were traded for 54 shares of IHS Markit RSU at merger. we had nothing to do with it and didn't get a dime

Because the deal was structured that way:

As set forth in the Form F4/Proxy you received:

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Tax Consequences to U.S. Holders of the Receipt of IHS Markit Common Shares

The receipt of IHS Markit common shares in exchange for IHS common stock pursuant to the merger will be a taxable transaction for U.S. federal income tax purposes to a U.S. holder of IHS common stock. In general, subject to the discussion below relating to the potential application of Section 304 of the Code, a U.S. holder will recognize gain or loss equal to the difference between (i) the fair market value of the IHS Markit common shares received by such U.S. holder in the merger (including cash in lieu of any fractional IHS Markit common shares) and (ii) its aggregate tax basis in the IHS common stock surrendered in the merger.

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It's exactly "as if" you got paid in cash for the stock you tendered and then took that cash and bought IHS Markit common stock with that cash.  You basis in the new stock is the same at the stock's fair market value you received.  Your holding period starts the day the deal settled.

When you receive these proxies or registration statements for proposed mergers you should read, if nothing else, the discussion in there about the income tax consequences to people who tender shares.

Tom Young