Hi Rahul,
Have you done some customizations to the navigation of the opportunity form?
Because there is only activity tab can be shown in the navigation of the OOB form, I can’t find task tab into the navigation?

For testing, i created one new 1:N relationship between Opportunity and Task, one task tab can be shown in the form now.


So you can go Settings > Customizations > Customize the system > Entities > Opportunity > Forms to open the main form.
Clicking 'Navigation' button and double-click 'Tasks' in the right pane to open 'Relationship Properties' dialog to check name.

Then go to relationships to open the relationship based on the name, then find lookup field.

If so, Task Associated view just show tasks related to opportunity based on the new lookup field value not regarding field.
Test:
Task:

Opportunity--01:

Opportunity--opp1:

Regards,
Leah Ju
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