Microcode is hard
In about 1981 a computer manufacturer - not Intel - trained me and a colleague (we were in an outside company) to create new microcode for their processor.
Microcode is hard. Components of the CPU are running in parallel, some taking several clock cycles, and the microcode designer must take account of these things.
I am sure modern Intel CPUs are much more complicated than anything from 1981. I hope they have better development tools, including simulators, than we did. But now I wonder if there are bugs or design weaknesses in those tools.
Or, I suppose, it could just be undue timescale pressures on the microcoders.