The Bug:
Mobs don't panic when hit with a summoned or dispensed potion of harming.
Steps to Reproduce:
Build the setup shown in the attachment below.
Obtain a potion of harming and place it inside the dispenser.
Summon a cow on the diamond block.
Activate the dispenser and observe the cow's behavior.
Take note as to whether or not mobs panic when hit with a summoned or dispensed potion of harming.
Observed Behavior:
Mobs don't panic.
Expected Behavior:
Mobs would panic.
Linked issues
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Can confirm as well; they don't seem to take knockback either.

Confirmed for 1.8.6.

Is this still an issue in the most recent versions (i.e. 1.10.2, or 16w42a) of Minecraft?
This appears to still be an active issue in 1.11.2. The command has changed due to Minecraft's updated entity handling, but the overall problem remains.
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Updated command:
/summon potion ~ ~5 ~ {Potion:{id:"minecraft:splash_potion",Count:1,tag:{Potion:"minecraft:harming"}}}
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I've also tried adding in various combinations of the "ownerName" and "UUID" tags to see if that solved the problem, to no avail:
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/summon potion ~ ~5 ~ {ownerName:USER,Potion:{id:"minecraft:splash_potion",Count:1,tag:{Potion:"minecraft:harming"}}}
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/summon potion ~ ~5 ~ {UUIDLeast:0L,UUIDMost:1L,Potion:{id:"minecraft:splash_potion",Count:1,tag:{Potion:"minecraft:harming"}}}
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/summon potion ~ ~5 ~ {UUIDLeast:0L,UUIDMost:1L,ownerName:USER,Potion:{id:"minecraft:splash_potion",Count:1,tag:{Potion:"minecraft:harming"}}}
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(Sorry about the strange spacing, I can't seem to get the formatting to look decent)

Confirmed for 1.12, the "fleeing" part also affects arrows and other projectiles.
Confirmed for 1.13.1.

Confirmed in 1.15.2 and 1.16 Pre-release 5.
I'd like to request the ownership of this report as the reporter has been inactive since October 2019.
Can confirm in 20w51a.
Can confirm in 21w06a.
This ticket relates to MC-44280.
Confirmed in 1.18 Release Candidate 2.
Fixed in 24w21a.
Can confirm.