In 2019, the DOL issued an opinion letter that said an employer OR an employee may not choose to waive or delay FMLA protection when the circumstance causing the absence is covered under the Act.
https://webapps.dol.gov/elaws/whd/fmla/overview.aspx
The FMLA prohibits employers from interfering with employees’ rights under the law and authorizes the U.S. Department of Labor (DOL) to investigate and enforce its provisions (See Investigative Authority). Employees cannot waive, nor may employers induce employees to waive, their prospective rights under the FMLA. This does not prevent the settlement of FMLA claims by employees based on past employer conduct without the approval of the DOL or a court. (See § 825.220 of the FMLA regulations.) Furthermore, nothing in the FMLA or its regulations prevents an employer from providing an employee with greater protections and/or more leave than entitled to under the law even if the employee is not, by law, eligible and/or the employer is not covered.
https://www.dol.gov/sites/dolgov/files/WHD/legacy/files/2019_03_14_1A_FMLA.pdf
OPINION
An employer may not delay the designation of FMLA-qualifying leave or designate more than 12 weeks of leave (or 26 weeks of military caregiver leave) as FMLA leave.
First, an employer is prohibited from delaying the designation of FMLA-qualifying leave as FMLA leave. Once an eligible employee communicates a need to take leave for an FMLA- qualifying reason, neither the employee nor the employer may decline FMLA protection for that leave. See 29 C.F.R. § 825.220(d) (“Employees cannot waive, nor may employers induce employees to waive, their prospective rights under FMLA.”); Strickland v. Water Works and Sewer Bd. of City of Birmingham, 239 F.3d 1199, 1204 (11th Cir. 2001) (noting that the employer may not “choose whether an employee’s FMLA-qualifying absence” is protected or unprotected by the FMLA). Accordingly, when an employer determines that leave is for an FMLA-qualifying reason, the qualifying leave is FMLA-protected and counts toward the employee’s FMLA leave entitlement. See 29 C.F.R. § 825.701(a) (“If leave qualifies for FMLA leave … the leave used counts against the employee’s entitlement ….”); WHD Opinion Letter FMLA2003-5, 2003 WL 25739623, at *2 (Dec. 17, 2003) (“Failure to designate a portion
of FMLA-qualifying leave as FMLA would not preempt … FMLA protections …”).3 Once the employer has enough information to make this determination, the employer must, absent extenuating circumstances, provide notice of the designation within five business days. 29 C.F.R. § 825.300(d)(1). Accordingly, the employer may not delay designating leave as
FMLA-qualifying, even if the employee would prefer that the employer delay the designation.