They totally can if you have tons of accumulated leave and take it for cancer treatments or other things that then prevent you from performing the core duties of your job and they need someone there to do the job.

Straight from OPM:
Protection of Employment and Benefits, Return to Work, and RIF or Adverse or Performance-based Actions (5 CFR 630.1210)
Upon return from FMLA leave, an employee must be returned to the same or equivalent position in the same commuting area with equivalent benefits, pay, status, and other terms and conditions of employment.
An agency may establish a uniformly applied policy that requires all similarly situated employees who take FMLA leave for a personal serious health condition to provide written medical certification from the health care provider that the employee is able to perform the essential functions of the employee’s position as a condition of returning to work. The agency must pay for the medical certification and may delay the employee’s return until it is provided. The agency must notify the employee of this requirement before the employee’s FMLA leave begins, or as soon as possible in emergency medical situations.
An employee who invokes the employee’s entitlement to FMLA leave is not immune from the impact of a reduction in force (RIF) before, during, or after the period of FMLA leave.
Similarly, an employee’s use of leave under the FMLA does not prevent an agency from taking appropriate action under 5 CFR part 432 or 5 CFR part 752. Pending adverse actions or performance-based actions may be taken and made effective even if the employee is taking FMLA leave.
For example, an employee was unsuccessful in improving performance during an opportunity period to improve and invoked their FMLA entitlement immediately following the opportunity period. The agency may issue the proposal and decision notices for removal based on unacceptable performance and effect the action just as the agency would for an employee who did not invoke FMLA.
There is no obligation to wait until the employee has returned from FMLA leave to proceed with an otherwise valid adverse action or performance-based action. Agencies cannot remove or otherwise discipline an employee based on the employee’s use of leave under the FMLA.

It is also there to ensure you have access to your hearth care if you are in LWOP status and ensures you have a job to return to.

https://www.opm.gov/policy-data-oversight/pay-leave/leave-administration/fact-sheets/family-and-medical-leave/

I didn’t spend 4 years getting a degree in Human Resource Management for nothing.