It’s a pick your poison situation. Reputation may have been damaged (who knows really, masses seem to forget rather quickly nowadays) but the alternative might just have caused a lot more trouble.
Just take a look at Fauci as an individual. He's never said masks don't work. He did say "there's no reason to be walking around with a mask," back in March, but, back in March, that was likely true. And, it's not like he hasn't reversed himself when new data comes in. That's precisely why he still has credibility: because what he says is always in line with the best available data.
Also I IMHO doubt that Fauci didn’t know about mask effectiveness - sure, maybe he didn’t know the very specifics but it was kind of obvious back then it wasn’t going to harm and one could infer that it’d probably help somewhat (nowadays it’s clear). IMHO all authorities just agreed to transmit a similar message around the world so they could in the meantime arrange more mask orders and sort out the distribution as production speeded up while avoiding massive riots and/or poor masks made at home that could’ve been worse than nothing.