It's not unheard of, but it is seen as aggressive on the part of the company and may lead to a strike, so it's used sparingly and only when the company feels it has a stronger hand.
I worked 10 years for an employer with multiple unions and a contentious relationship with each of them. (I was in one union but there were at least 3 other unions working at my site). I don't recall them "surprising" anyone by making the contract worse but I think every single one had some kind of "signing bonus" that went away if it wasn't ratified, and that would often disappear or be reduced in exchange for a concession (which was usually maintaining benefits). The company would then point to that extra money as "lost" by the union when encouraging other sites not to unionize.
(ETA, an example would be a $5k signing bonus going away and being replaced by lower medical premiums and additional parental leave, which the union had come in asking for. For employees with single insurance and no plans for future children, the $5k might have been a better deal, but the overall union wanted the family benefits, and in any case the signing bonus was removed from the final voted contract, which the company would tout as a "loss" for the union.).
It's not common, but it does happen. The members could vote the worse one down too, but the Employer has the right to implement it. So usually when the union members vote down the crappy vote knowing the Employer will probably implement it, the next step taken at that same meeting is to conduct a vote to strike.
"Why would you ruin perfectly good peanuts by adding candy corn? That's like saying hey, I have these awesome nachos, guess I better add some dryer lint." - Nonny
This happened with the Frito Lay negotiations last year. What they ended up agreeing to was worse than the initial offer. They went on strike, but I think it was difficult for workers to do it for long due to the low union weekly pay while on strike. I believe many people started crossing the picket line pretty early on. Their biggest complaint other than wages was forced overtime. I could possibly see a better outcome this year though as the labor market is even worse and there is probably more pressure on the company. There’s a decent amount of competition here with several large retailer DCs and manufacturing plants. Almost all had higher starting wages than Frito Lay.