Reproduction steps:
1. Open project in "ButtonHighlight.zip (25.8 KB)"
2. Open SampleScene and enter Play Mode
3. Highlight the button by hovering the mouse above it (it will turn red while the mouse is hovering above it)
4. Click the button once
5. Try hovering your mouse above the button to highlight it

Expected result: button gets highlighted (it will turn red while the mouse is hovering above it)
Actual result: button no longer gets highlighted

Reproducible with: 2019.4.10f1, 2020.1.4f1, 2020.2.0b1
Could not test with: 2018.4.27f1, 2019.3.0a1, because after pressing it the button always stays highlighted

Notes:
1. If switching Unity versions be sure to create a new Event System and Canvas (including its all elements) (older bugs remain after updating the project)
2. It seems this issue was introduced when fixing the permanently highlighted button so I will not mark this as a regression, I could not find however find the version where the change was made (somewhere in 2019.1, 2019.2 alphas)
3. Also reproducible in build (Standalone)
4. Video is attached