Steps to reproduce:
1. Open the attached project ("1324518.zip")
2. Open "SampleScene"
3. Build to a device
4. Press one button
5. While releasing the pressed button, press the other button

Expected results: OnPointerDown is triggered whenever a button is pressed
Actual results: OnPointerDown is sometimes not triggered when a button is pressed shortly after releasing another button

Reproducible with: Input System 1.1.0-preview.2, 1.1.0-preview.3 (2019.4.24f1, 2020.3.4f1, 2021.1.3f1, 2021.2.0a13)
Not reproducible with: Input System 1.0.0, 1.0.1, 1.1.0-preview.1 (2020.3.4f1)

Reproducible on:
iPhone XR (13.4.1)
VLNQA00013, Samsung Galaxy S6 Edge Plus (SM-G928F), Android 7.0, CPU: Exynos 7 Octa 7420, GPU: Mali-T760
VLNQA00122, Samsung Galaxy S9 (SM-G960F), Android 10, CPU: NOT FOUND, GPU: Mali-G72
VLNQA00331, Huawei P20 lite (ANE-LX1), Android 9, CPU: HiSilicon Kirin 659, GPU: Mali-T830

Notes:
- The case seems closely related to Case 1267026 (https://issuetracker.unity3d.com/product/unity/issues/guid/1267026/), as the behaviour in that case can be triggered following the steps from this scenario. However, The mentioned issue doesn't seem to reproduce from 1.1.0-preview.3, only this issue remains.