Here is a similar thread, as mentioned:
When we first invite the external users to access SharePoint Online site, an invitation is sent to them via email which contains a link to the site, they must use the link to log in and verify their identity. Once they have accessed the site for the first time, these external users are added as guests to our Office 365 directory. In this case, the “Send an email invitation” option is mandatory.
Once they are in the user list in your Office 365 subscription, we can grant them access to other sites or documents without sending them additional invitations. In this case, we can uncheck the “send an email invitation” option when share with external users. Please note we need to wait some time after sharing with external users for the first time, then share other SharePoint sites in the same tenant without emails.
External sharing overview