Searching in shared mailbox of some users stop working

Hi, 

we have problem with searching in shared mailbox. It works without any problems. But 1 week ago, two users started problem with searching. Not on the same shared mailbox. Another users still searching without any problems. 

One user have many shared mailbox, but searching stopped only of one of them. 

I tried reindex mailbox of users, it seemed that it works, but after few minutes the problem was again

I've read thist article Outlook Not Searching Exchange Shared Mailbox - Microsoft Q&A and I think this problem was and is up to date

One user use BS and Office 365 desktop Outlook.
Another user use BB and Outlook 2019

Do anyone experience with this issue? And can give some advices how could resolve this unpredictable problem

Thanks