If you have the money withdrawn each pay period, it also reduces the amount you owe on Medicare and Social Security taxes. By paying the FSA in full in 6 pay periods, you're missing out on 20 pay periods of reduction in Medicare/SS; you don't recover those at tax time if you pay upfront and lose out on those savings entirely.

https://www.fsafeds.com/support/faq/all/9

See the second paragraph:

Further, money set aside through an FSA is also exempt from FICA (Social Security and Medicare) taxes. This exemption is not available on your federal income tax return. However, you may wish to consult with a tax professional to determine which option is best for you.