Repro:
1) Create new URP (or HDRP) project.
2) Add terrain.
3) Bake (you can reduce resolution for faster bake times).
4) When bake is complete, select the "Baked Lightmap" debug visualization. You should see the lightmap data.
5) Go to the terrain's settings and enable "Draw Instanced". At this point the lightmap data disappears (only wireframe remains).

Expected: The lightmap data is shown even if I enable "Draw Instanced".
Actual: It is not shown.

In projects using Builtin/Legacy renderer, this works fine. It only appears in URP/HDRP. It seems this was even an issue when instanced terrain drawing was introduced in 2018.3 (tested with 2018.3.6f1).

This issue is probably related to https://fogbugz.unity3d.com/f/cases/1082011/. I guess that the fix for 1082011 solved the problem for Builtin/Legacy but did not look at URP/HDRP (but this needs verification).

****************** Old repro steps ******************
Steps to reproduce:

1. Download and open FontaineBleu project (link in the comments)
2. Navigate to Lighting Settings
3. Bake the lighting
4. While the lighting is baking (or has finished), switch to Baked Lightmap scene view mode
5. Notice that terrain is not visible

Notes:

- Does not affect built-in RP and URP
- Does not reproduce in template projects with a flat terrain added to them