the ruling of centaurs and lances is as silly as the shield bash errata. Neither of them make any sort of sense. Shield Bash has received no errata.
If a DM would like a centaur to be considered on a mount all the time, the DM is free to make that decision. But by rule, the centaur isn't permanently mounted.
— Jeremy Crawford (@JeremyECrawford) February 21, 2019