main/dhcpcd-7.0.6-r0: Deletes Link Local addresses when assigned to multiple VLAN interfaces
Say you have vlan2, vlan3, vlan4. dhcpcd will delete the LL from vlan2 and vlan3 when the system is booting, leaving only a LL for vlan4. This is because they have the same MAC address and thus the same LL.
A patch from upstream. Has been committed to their git repository. Will appear in next release. https://patchwork.alpinelinux.org/patch/4016/
I have tested it and it works, solving this problem.