Found the solution. Jenkins environment variable was messing with the kernel release string. You can change this behaviour in the main kernel makefile:
# KERNELRELEASE can change from a few different places, meaning version.h
# needs to be updated, so this check is forced on all builds
uts_len := 64
ifneq (,$(BUILD_NUMBER))
UTS_RELEASE=$(KERNELRELEASE)-ab$(BUILD_NUMBER)
else
UTS_RELEASE=$(KERNELRELEASE)
endif