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My daughter had a similar problem.  She is single and claimed 2 allowances on her W4 back in 2018.  She had federal withholding on most of her wages in 2020 and then toward the end of the year it went to zero.  We didn't notice because she got a refund.  

This year (2021) no federal taxes were withheld from her income and all of a sudden she owes a lot.  I did some digging based on questions she asked HR.  They said that her withholding was based off of the new W4 from 2020, even though she never filled out one as was not required to.  I found this from the IRS, which was implemented to make it easier on employers.  If I read this correctly, based on number 3 below, they multiplied her allowances (2) by $4,300.  So, they are saying line 4(b) would now have $8,600 in deductions to withholding.  Could this have caused no federal withholding?   She make 43,000 a year and she owes $3,00.  This was totally unexpected and will be a burden for her.  

How To Treat 2019 and Earlier Forms W-4 as if They Were 2020 or later Forms W-4

Employers may use an optional computational bridge to treat 2019 or earlier Forms W-4 as if they were 2020 or later Forms W-4 for purposes of figuring federal income tax withholding. This computational bridge allows you to use computational procedures and data fields for a 2020 and later Form W-4 to arrive at the equivalent withholding for an employee that would have applied using the computational procedures and data fields on a 2019 or earlier Form W-4. You must make up to four adjustments to use this computational bridge.

1. Select the filing status in Step 1(c) of a 2020 or later Form W-4 that most accurately reflects the employee’s marital status on line 3 of a 2019 or earlier Form W-4. Treat the employee as “Single or Married filing separately” on a 2020 or later Form W-4 if the employee selected either “Single” or “Married, but withhold at higher single rate” as their marital status on their 2019 or earlier Form W-4. Treat the employee as “Married filing jointly” on a 2020 or later Form W-4 if the employee selected “Married” as their marital status on their 2019 or earlier Form W-4. You can’t convert an employee to a filing status of “Head of household” using this computational bridge.
2. Enter an amount in Step 4(a) on a 2020 or later Form W-4 based on the filing status that you determined in (1) above when you converted the employee’s marital status on a 2019 or earlier Form W-4. Enter $8,600 if the employee’s filing status is “Single or Married filing separately” or $12,900 if the employee’s filing status is “Married filing jointly.”
3. Multiply the number of allowances claimed on line 5 of an employee’s 2019 or earlier Form W-4 by $4,300 and enter the result in Step 4(b) on a 2020 or later Form W-4.
4. Enter the additional amount of withholding requested by the employee on line 6 of their 2019 or earlier Form W-4 in Step 4(c) of a 2020 or later Form W-4.

.This computational bridge applies only for Forms W-4 that were in effect on or before December 31, 2019, and that continue in effect because an employee didn’t submit a 2020 or later Form W-4. If an employee is either required, or chooses, to submit a new Form W-4, it doesn’t change the requirement that the employee must use the current year’s revision of Form W-4. Upon putting in effect a new Form W-4 from an employee, you must stop using this computational bridge for the applicable year of the new Form W-4. An employer using the computational bridge for a Form W-4 furnished by an employee must retain the Form W-4 for its records..