I login to portal https://app.ringcentral.com/login on Chrome (incognito mode) with my credentials (sadish@tealjones.com – test account) and MFA (enabled for all users), then open another tab visit www.office.com and click sign in. Here without authentication or MFA I am able to login to the M365 portal. Now I go to the first tab (where I was logged in to RingCentral) and log-out. Then Sign back in again, at this point RingCentral allows me to the portal without password or MFA. Now, while logged on the RingCentral portal (1st tab), I logout from M365 (2nd tab), then go back to RingCentral tab and log-out. Now, when I try to re-login, it asked for password and MFA.

Which means when you logout of RingCentral the logout request is not passed back to Entra ID.

This is related to Single Logout (SLO) for Single Sign-On (SSO). We need this in this day of age.