Guests cannot add comments to Planner

[Sarnst]

We have external users with a mailbox and an E3 licence. 
with their external email address they have also been added as a guest user to Teams to make it easier to work in the Teams App and in their own company.
Somehow the guests are added as a mailcontact in exchange.
They are not able to add comments in planner.
This only applies for 2 users all others in the same situation are fine and normally added as guests

Trying for 3 months to get it fixed with MS support and they have no idea.

[#2]

@Sarnst Does the guest users able to post replies for the conversations directly from group mailbox?

[#3]

@Santhosh Balakrishnan Hi, we face the same issue, we are guests user in one of our client's planner and we can not add comments in tasks. However we can edit the Note section.

Any news on the issue?

Thank you!

[#4]

did you found any solutions ?

[#5]

Hello @Sarnst,
I know this post is very old. Are you still having this issue? As I have a user with the same issue, but cannot replicate it on a test account. Are you saying the problem occurs specifically if the external user has a contact in Exchange?
I've done a little bit of reading and 'Comments' depend on the user having and Exchange mailbox.

https://support.microsoft.com/en-us/office/comment-on-tasks-in-microsoft-planner-fd4aedde-7785-4cd0-...
Is it possible that that having a Contact entry stops planner being able to identify their external mailbox?
Thanks
Laurence