When you jump, and hit a mob/player (while having a negative velocity), you do a critical hit, hence dealing more damage.
There is a potion effect called strength, and when you critical hit someone, the strength potion's extra damage gets overrode by the critical hit's damage.
Thank you for your report!
We're actually already tracking this issue at MCPE-47766, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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