After fixing water damage: Apps accessing stored data fail or crash

About a year ago I bought a cheap iPhone 6s with water damage to give to my aunt after repairing it. The phone works fine overall but some weird issues prevail.

What I did:

I replaced the following parts:

• Screen
• Battery
• Dock Connector

I cleaned the logic board with cleaning alcohol, but I did not submerge it. Also I did not clean under covered chips.

Issues that were experienced:

The phone works fine overall (for almost a year now) e.g. installing apps, taking pictures etc. However it:

• gets very hot while charging
• the photos app crashes when opening a photo
• the photos sometimes shows less photos than there are
• iOS-updates don’t work unless you disconnect the screen panel (and keep the homebutton and digitizer connected)

My question:

Could this be caused by a faulty NAND storage chip? Are there any steps I can try for fixing this? It seems that most read and write operations work fine.

Thanks for your assistance.