Why is the tooltip layer of ngx-bootstrap covered by the following content?

I introduced tooltip into my calendar and found that the following content will cover tooltip , but z-index of tooltip is already the highest. What"s the problem? How to solve it?

Mar.18,2021
The z-index of the

child element is affected by the parent element. If there are two elements An and B, if the child element z-index of An is 999 but the z-index of An element is 1, the z-index of B is 2, then the child element of A will still be covered by B

.
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