After looking at the code and doing more tests I can provide a little bit more information about this issue.

After first login using google and while keycloak still has the google refresh token I can call this endpoint without issue. Even after 1 hour the token is well refreshed by keycloak without loosing the google refresh token :

        String baseUrl = keycloakUrl + "/protocol/openid-connect/token";
        MultiValueMap<String, String> requestBody = new LinkedMultiValueMap<>();
        requestBody.add("client_id", keycloakClientId);
        requestBody.add("client_secret", keycloakClientSecret);
        requestBody.add("grant_type", "urn:ietf:params:oauth:grant-type:token-exchange");
        requestBody.add("subject_token_type", "urn:ietf:params:oauth:token-type:access_token");
        requestBody.add("subject_token", accessToken);
        requestBody.add("requested_issuer", "google");

But if I logout from my app and i log in again using google the refresh token is lost forever.

This feature is used to sync calendar between my app and the user google agenda. If someone can provide me with some information about where to look I may be able to fix it but right now it seems too complicated for me.

A little more digging on the code.
It seems to be well handled in OIDCIdentityProvider.exchangeStoredToken, the refresh token is not overrided if response doesn't contain one :

                if (newResponse.getRefreshToken() == null && tokenResponse.getRefreshToken() != null) {
                    newResponse.setRefreshToken(tokenResponse.getRefreshToken());
                    newResponse.setRefreshExpiresIn(tokenResponse.getRefreshExpiresIn());
                }

I think the same should be done in IdentityBrokerService.updateToken().

I may be able to make a PR

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