Usually, sprint doesn’t work in the game I’m working on. Sometimes, if I tweak the Island sprint settings, it will work once, but not the next time.
This is weird, because I actually have a different UEFN game already public where sprint always works. But I can’t figure out the difference between the two.
I’m wondering if others have had this issue and found a way to fix it.
Just happened to me, I forgot to activate the “Allow Sprinting” in my “Class Designer” device. There may be more devices that allows you to quit sprinting. I can’t think on anything else, good luck 
It’s been a while, but has there been a fix to this? I’m running into the same exact issue, and it also affects sliding as well.
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For more information, such as how to get the reference ID, please check out the article here: Using the Creative and UEFN Bug Reporting Form