Reported on mqtt-comment by Christoph Krey:

I am referring to the Fixed Header illustration of the CONNACK packet [at line 1321]

It shows the remaining length as fixed = 2 rather than >= 3 because the Properties will follow in V5 at least with a zero property length.

This is why you should amend figure 3-7 and call it nonnormative example similar to the figure used for PUBLISH or other Control Packets:

1902 Figure 3-9 - PUBLISH packet Variable Header non-normative example