Currently $wgOAuth2GrantExpirationInterval has a default value of "PT1H" (one hour) and $wgOAuth2RefreshTokenTTL has a default value of "PT1M" (one minute).
Having refresh tokens expire so much faster than access tokens makes them almost completely useless. Therefore I would like to suggest changing the default value of $wgOAuth2RefreshTokenTTL to something longer like for example "P1M" (one month).