Repro:
1) Be sure to have iOS support installed.
2) Create new project and set build target to iOS.
3) Generate standard reflection probe by pressing "Generate Lighting" in Lighting window.
4) With the generated reflection probe visible in Inspector, change the Lightmap Encoding.
5) Observe the unexpected message "Not yet compressed" in the bottom of TextureImporter.

Expected: You can change Lightmap Encoding without ending up in the "Not yet compressed" state.
Actual: You end up in the "Not yet compressed" state.

The repro was done using 2019.4.15f1 on MacOS.