Are you trying to use as a static address one which falls inside the router's address pool? If so, there is no telling what your router will do. Check the router address pools from the GUI, and pick an address outside the pool.

You may check whether you still experience such phenomena simply by stopping network manager (I presume you use one), giving yourself an address,

  ip link set dev eth0 down
  ip addr add 192.168.1.whatever/24 dev eth0
  ip link set dev eth0 up
  ip route add default via 192.168.1.1

without using dhcpcd. This has the advantage of isolating the problem: if it happens again, then it's your router's fault, otherwise it was an incorrect choice of static IP address.

Are you trying to use as a static address one which falls inside the router's address pool? If so, there is no telling what your router will do. Check the router address pools from the GUI, and pick an address outside the pool.

You may check whether you still experience such phenomena simply by stopping network manager (I presume you use one), giving yourself an address,

  ip link set dev eth0 down
  ip addr flush dev eth0
  ip addr add 192.168.1.whatever/24 dev eth0
  ip link set dev eth0 up
  ip route add default via 192.168.1.1

without using dhcpcd. This has the advantage of isolating the problem: if it happens again, then it's your router's fault, otherwise it was an incorrect choice of static IP address.