Create a custom block programatically. Let's call it `CustomBlock`.
Add `getCacheTags()` method to it and set some cache tags.
Use this block in quicktab.
Verify if the quicktab block contains the cache tags of `CustomBlock`. This can be verified from the `cache_render` table where `cid` column will contain the id of the quicktab block.
It will be seen those cache tags are not present.
This causes problem with cache invalidity.
Proposed resolution
Consider the cache metadata set at block level and merge it to the cache metadata of the quicktab block.