The new kernel is quite large and we should keep some extra space for future updates and kernels.
https://git.ipfire.org/?p=ipfire-2.x.git;a=commitdiff;h=bea1884bd79cd91c4b31c2bd20c6f06d029c1e22
@Arne: Do you want to align the size of the /boot partition in the flash image?
Core Update 122 has been shipped; I assume this is fixed. If it is not, please reopen.