Google "taxation of VEBA withdrawals" and you'll find a lot, including this, which indicates that distributions for qualified medical expenses should not be taxable. However, just because distributions were reported to IRS does not mean they were reported as taxable - double check your 1099. If you still have questions, contact the plan administrator, which should be listed in your Summary Plan Description. Hope this helps, good luck.
https://www.investopedia.com/terms/v/voluntaryemployeesassoc.asp