I have wpnav set to 600 cm/s and rtl speed set to 0 (so it should use wpnav).
During rtl in auto mission, they both are ignored and quad accelerates up to 1000 cm/s.
I tried to set rtl speed to 600 cm/s instead of 0 but it too gets ignored.
AC 4.0.7 on orange cube.
UPDATE, my bad, i added a do set speed of 1000 cm/s on the script that modifies my missions and sets some parameters we need in each mission. There is indeed a do set speed of 1000 cm/sec, so all is good.