Shared mailboxes gets locked out

We have a mysterious problem where our shared mailboxes gets locked out. The origin is always Outlook on some of the users computers. I can see in the eventlog there are 4648 events recorded similar to these:

A logon was attempted using explicit credentials.

Subject:
    Security ID:        privdomain\xuserx
    Account Name:       xuserx
    Account Domain:     privdomain
    Logon ID:       0x76CBE
    Logon GUID:     {00000000-0000-0000-0000-000000000000}

Account Whose Credentials Were Used:
    Account Name:       sharedmailbox@privdomain.com
    Account Domain:     privdomain
    Logon GUID:     {00000000-0000-0000-0000-000000000000}

Target Server:
    Target Server Name: ex01.privdomain.com
    Additional Information: ex01.privdomain.com

Process Information:
    Process ID:     0x100c
    Process Name:       C:\Program Files (x86)\Microsoft Office\root\Office16\OUTLOOK.EXE

Network Information:
    Network Address:    10.20.30.40
    Port:           443

This event is generated when a process attempts to log on an account by explicitly specifying that account’s credentials.  This most commonly occurs in batch-type configurations such as scheduled tasks, or when using the RUNAS command.

What I have problems understanding is how Outlook lock out the mailbox user, when the users are logging on to the mailboxes with their own user credentials? They use the add account feature from within Outlook, where they add the email address of the shared mailbox and the shared mailbox is added as an aditional mailbox without any question about username or password. Both user and shared mailbox are on on-prem Exchange 2016, while Outlook is current 365 app. The users locking out the shared mailboxes are not getting locked out them selves.

They do add the mailbox similar to the following blog, but they are not prompted for a username and password: https://www.michev.info/Blog/Post/2025/how-to-add-a-shared-mailbox-as-additional-account-in-outlook-2016-build-8431-and-above-on-windows-10-1709-and-above

We also have a fair bit of users getting locked out, seemingly from the usermanager service on their computers, but we are unsure if it is related somehow.