CXXRTL: always_ff triggered even though no sensitivity condition is met

How to reproduce:

bash run_cxxrtl.sh

Yosys version: Yosys 0.37+21 (git sha1 3d9e44d18, clang 10.0.0-4ubuntu1 -fPIC -Os)

Expected:

out 0 = 0
out 1 = 0
out 2 = 0
out 3 = 0

Actual:

out 0 = 0
out 1 = 0
out 2 = 0
out 3 = 1

For comparison, the same code in Verilog produces the expected result:

bash run_verilator.sh