On February 28 Collins and the NTSB were actively testing for freezing of moisture ingression as the cause, having done some work earlier in the month. (Source is NTSB’s press release, not its report.)
Collins did give Boeing a list of affected actuators in August.
Between those dates we don’t know all that was done, but AWST says the NTSB issued a preliminary report six months ago which is roughly March or April. I CORRECTED WHAT I SAID, I’ve confused the preliminary report with the recent safety recommendations.
I haven’t seen Boeing’s rumoured MOM from March. An MOM alerting all operators to the moisture problem would motivate good operators to check their aircraft. (I don’t now how visible the bearing is insitu - appears to be on the bottom of the unit.)
NTSB now says FAA did little, but that claim seems to only relate to NTSB’s recent preliminary report. https://aviationweek.com/air-transport/safety-ops-regulation/ntsb-chair-disappointed-faas-response-737-rudder-issue