Yes, sometimes employers do things, not realizing the negative effects it can have on employees.
In your case, because the check was dated 1/4/2019, you can make a good argument that you did not have "constructive use" of the money until 2019 (this is the usual way you determine to what year an income or expense item should belong).
Yes, if you want to dispute this (see below), you would need to amend your 2018 return.
However, you cannot use their W-2 to file the return you want. You will have to use form 4852 (see this IRS page). This is permitted when your employer will not give you a correct W-2.
At the bottom are the instructions inside TurboTax on how to enter the 4852. Note that you will enter your W-2 information as you believe it to be accurate (without the bonus amount), then check on the next screen that you want to do a substitute W-2.
You will need to explain why you have made this change (i.e., why the employer reported your W-2 incorrectly) and what you did about it (repeatedly contacted the employer who refused to correct the W-2).
Note that this will give you the return you expected, but there may be push-back down the road. The IRS may contact the employer for an explanation. So be sure you want to do this. But, like I said, you didn't have constructive use of the money in 2018 (indeed did you even know about the bonus in 2018?), so reporting the income in 2019 would be more in keeping with the spirit of the Tax Code.
If you get a letter from the IRS, you may want to consult with a local Enrolled Agent or CPA. This issue probably doesn't rise to the level of a tax attorney, although they could certainly handle it. Or you may want to talk to a local professional now, to advise you on how to proceed.
In any case, this is not tax advice to you, it is a description of how you would proceed if you want to contest this W-2.
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Here's how to fill out Form 4852:
1. Visit the W-2 area of TurboTax and choose "Work on my W-2 now."
2. Enter the employer ID Number (EIN) if you know it. Leave it blank if you don't.
3. When we ask for your W-2 info, enter all the info that should be on your W-2 if you had actually received it. You can use your pay stubs to estimate your earnings and total taxes paid (federal, state and local). Because you don't have your actual W-2, you just need to use the best information you have available (pay stubs, bank statements, etc.) to fill out as much of the Form as possible.
4. After you're done filling out your W-2 info, you will get to "Do any of these uncommon situations apply to this W-2?" Select the checkbox "Substitute form -- I didn't receive a W-2 from my employer and need to complete a substitute form."
5. Then we will ask you to confirm you want to complete form 4852. Select Yes if you are ready.
6. Next write out an explanation of how you figured out your wages and taxes paid (i.e. I used my last pay stub or my most recent bank statement).
7. Then describe the steps you took when you tried to get your W-2 from your employer, like I called my employer on (insert date).
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