Yep has happened occasionally for me as well on multiple phones. I just hit the train voice setting and it says "you already have a voice recorded, you're all set" and it's back to working till the next time an update breaks it
This is a constant frustration for me and I feel like it's been going on for years. There are two settings that won't "stick". First is from Google settings, Settings>Google Assistant>Assistant voice & sounds>Speech output>Full. Second is Voice>Spoken results>All voice searches. Setting these will work for a while then they reset on their own.