They start apps, race through photos, type stuff... Like an angry ghost letting off steam.

BOTH devices exhibit this bizarre behavior following repair i Vietnam.

 

WTH was done? Were parts removed and replaced? What is causing this erratic behavior?

Genuine parts from a genuine reseller?  

9 minutes ago, Kf6vci said:

Were parts removed and replaced?

You said that yourself in title.  As asked above was this an official repair service?

My guess would be an aftermarket screen that has a defect ( or not installed properly ) and causes the screen to send "touch-signals" to the operating system.

When I had a cracked screen once and would push the bottom right corner, it would trigger the push in the top left corner.

21 hours ago, lopburi3 said:

As asked above was this an official repair service?

I'll assume the OP's reticence signifies a big fat no, particularly as he's been online within the last hour.

 

When seeking Apple repairs it's a good investment to go straight to Apple for a quote as they can be surprisingly understanding and generous when out of guarantee.  They're obviously expensive products and with that requires genuine replacement parts for quality assurance.  This also helps when selling the product on as have a good resale value if all is in order.  

On 12/5/2019 at 8:43 PM, torturedsole said:

Genuine parts from a genuine reseller?  

I don't know - but the issues speak for themselves. BTW, a glass screen replacement by an authorized dealer worked 100% and the charge was less than $ 40 for an Ipad Mini.