This bug has been around for 5 years, as in the post below
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https://forums.unrealengine.com/t/survey-theres-a-really-really-terrible-bug-where-child-blueprints-get-their-variables-reset/90578
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I also encountered it when I first learned UE this year, but I didn’t know what happened at that time.
But this time I isolated the buggy assets, and here is my project, along with a video of how to reproduce the bug.
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https://1drv.ms/u/s!AtEph48WiKrhjmTNGxHj3txU57Ug?e=OaH0zn
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Although my project is 4.26.2, I have tested 4.27.2 and UE5 and the bug still exists.