SharePoint External Permissions

I am using SharePoint online (365), and am having an issue inviting 'external' users to a site collection. A few months ago, inviting users to the site worked without issue. We were able to share the site with multiple users on different domains, and each was able to view with the proper permissions they were assigned (invited from a group in "People & Groups"). I recently encountered an issue using the "Share" command, or trying to add users to groups. Doing so resulted in a pop-up error "Could not resolve memberid, (followed by the user i was inviting)"; after clearing the browser cache, this issue ceased, and I was able to invite people without getting any errors. The user would show up in the SharePoint groups & Office 365 "People" as 'user_domain_com#ext#sharepointdomain.onmicrosoft.com' after sending the invitation, and the link would send to their email; however when they use the link, they get the following:

If I invite 'user@domain.com', the message is:

"Access denied, the user 'user_domain_com#ext#sharepointdomain.onmicrosoft.com' does not have permission to view the site."

The user's email, name, and info seems to be correctly adding to the groups I assign & invite them to. External sharing is turned on at the top site collection level, and on the site itself. I have checked both via the SharePoint admin center, and the "Site Settings". Additionally, Azure AD shows them as "Users" with their proper email addresses.

Thanks