Steps to reproduce:
1) Open Unity Hub and create a new URP template project
2) Open Build Settings and switch platform to Android
3) Make sure that Gles3 is the only Graphics API in the list
4) Create Canvas in the scene and add an image
5) Apply provided image
6) Build and Run project on a device

Observe darker UI (check attached image "repro.png")

Reproducible with:
2019.1.0b6, LWRP 5.6.1
2019.1.0b9, LWRP 5.7.2
2019.1.0b10, LWRP 5.7.2
2019.1.0f1, LWRP 5.7.2
2019.1.12f1, LWRP 5.7.2
2019.2.0f1, LWRP 6.9.0
2019.3.1f1, URP 7.2
2020.1.0a22, URP 7.1.8

Not reproducible with:
2018.4.16f1, LWRP 4.1.0
2019.1.0b1, LWRP 5.2.3
2019.1.0b5, lwrp 5.2.3, 5.3.1, 5.6.1

Regression since:
2019.1.0b6

Devices under testing:
Reproduced with:
VLNQA00104, Lge Nexus 5 (Nexus 5), Android 6.0.1, CPU: Snapdragon 800 MSM8974, GPU: Adreno (TM) 330
VLNQA00023, Xiaomi Mi 4i (Mi 4i), Android 5.0.2, CPU: Snapdragon 410 MSM8916, GPU: Adreno (TM) 405
VLNQA00058, Sony Xperia Z (C6603), Android 4.4.4, CPU: Snapdragon S4 Plus MSM8960, GPU: Adreno (TM) 320
VLNQA00324, Sony Xperia T2 Ultra (D5303), Android 5.1.1, CPU: Snapdragon 400 MSM8226, GPU: Adreno (TM) 305

Not reproduced with:
VLNQA00217, Razer Phone 2 (Phone 2), Android 9, CPU: Snapdragon 845 SDM845, GPU: Adreno (TM) 630
VLNQA00123, Google Pixel 2 XL (Pixel 2 XL), Android 10, CPU: Snapdragon 835 MSM8998, GPU: Adreno (TM) 540
VLNQA00134, Samsung Galaxy S8 (SM-G950F), Android 9, CPU: Exynos 9 Octa 8895, GPU: Mali-G71

Note:
Not reproducible using Built-in RP
Not reproducible using Gamma color space