How to reproduce:
1. Open the attached project
2. Build and run keyboard-type scene
3. Click on Input field (where Default is written)
4. When keyboard opens, type a word then open emoji and click on one
Result: you can delete text and write a new one, in this case input field updates
5. Now type two or more emojis
6. Try to delete what is written and write something new
Result:Text in UI Input field doesn't update

Note:
I have tried with all keyboards (reproducible with all where it is possible to type emojis)
Also tried with Native keyboard input field, issue doesn't reproduce

Reproducible: 2017.4.7f1, 2018.2.2f1, 2018.3.0b7

Tested and reproduced:
VLNQA00099 Google Pixel XL 9 Snapdragon 821 MSM8996 Pro Adreno (TM) 530 OpenGL ES 3.2
VLNQA00122 Samsung Galaxy S9 8.0.0 Exynos 9 Series 9810 Mali-G72 OpenGL ES 3.2
VLNQA00004 Meizu 5.0.1 MediaTek Helio X10 MT6795T PowerVR Rogue G6200 OpenGL ES 3.1 build 1.4

Tested and didn't reproduce:
iPhone 8 plus iOS 11.0