Steps to reproduce:

1) Download attached project 'ButtonInScrollView.zip' and open in Unity
2) Build and run 'ReproScene' on a device
3) Press on button 'Press1'
Note: There should be text output on the left side that first button was pressed
4) Press button 'Press2'
Note: There should be text output on the left side that second button was pressed
5) Keep pressing those buttons and eventually you will notice that sometimes text did not changed

Expected result: After button was pressed there should be an output in Text component, and show which button was pressed
Actual result: Sometimes there is no output after button was pressed

Reproduced with:
5.3.7p4, 5.4.4p4, 5.5.2f1, 5.6.0b10

DUT:
Reproduced with:
Samsung SM G930F (Galaxy S7), OS:7.0, CPU:arm64-v8a, GPU:Mali-T880, Build:samsung/heroltexx/herolte:7.0/NRD90M/G930FXXU1ZPK3:user/release-keys
Samsung SM G925F (Galaxy S6 Edge), OS:6.0.1, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zeroltexx/zerolte:6.0.1/MMB29K/G925FXXU5DQA7:user/release-keys

Not reproduced with:
Samsung GT I8190, OS:4.1.2, CPU:armeabi-v7a, GPU:Mali-400 MP, Build:samsung/goldenxx/golden:4.1.2/JZO54K/I8190XXAMG4:user/release-keys
Lge LG D855 (G3), OS:5.0, CPU:armeabi-v7a, GPU:Adreno (TM) 330, Build:lge/g3_global_com/g3:5.0/LRX21R.A1445306351/1445306351:user/release-keys
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Fixed in: 2018.1.0a1