A fresh installation of a UEFI machine creates an ESP partition with FAT16 as
boot filesystem:

# parted
GNU Parted 3.3
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print                                                            
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 68.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  525MB   524MB   fat16                 boot, esp
 2      525MB   66.6GB  66.0GB  btrfs
 3      66.6GB  68.7GB  2149MB  linux-swap(v1)        swap

According to the UEFI specification it should be FAT32.
https://uefi.org/specifications
"EFI encompasses the use of FAT32 for a system partition, and FAT12 or FAT16
for removable media."

Although UEFI firmware must handle FAT12, FAT16 and FAT32 file systems, I
recommend to use FAT32. (Just to follow the rules. I don't expect any error
cases with FAT16. It just looks like an error.)