As shown in attached android-lm-artifacts.png, regular lightmaps (i.e. not directional) show compression artifacts when Build Target = Android, Lightmap Encoding = Low.

Tested on Unity 2021.1.0a7.

Steps to reproduce:
1) Make sure to have Android support installed.
2) Open attached project.
3) Open SampleScene.
4) Observe that there is no lightmap artifacts.
5) Select Android as build target.
6) Observe that there's now lightmap artifacts.

Expected: No or barely visible compression artifacts.
Actual: Clearly visible compression artifacts.

Note that the behavior is slightly different depending on whether Color Space = Gamma or Linear. If Color Space is Gamma then Unity chooses the ETC1 format. If Color Space is Linear then ETC2 is chosen. However, regardless of Color Space, the same artifacts still show up which is arguably somewhat surprising given how Unity says it the two cases use different compression formats.