There will be changes in the next release specifically related to the boosted hero by default in the current event banner's pity mechanism.
Once you successfully pull the hero boosted by default for the second time, the default boost for that hero will be removed. Consequently, the drop rate and booster calculation for the banner will adjust, leading to higher overall chances of obtaining the other legendary heroes in the pool.
▻ Note: After the default booster is removed, boosters will behave identically to the one in the base banner.
We will announce the official dates once everything is finalized. Stay tuned for more updates!