Reproduction steps:
1. Open "case_1170597-ETC2_fallback_system_legacy_packer.zip" project
2. Enter Play mode
3. Notice "Size: 21MB"
4. Exit Play mode
5. Change The "Sprite Packer" "Mode" from "Always Enabled(Legacy Sprite Packer)" to "Always Enabled"
6. Enter and Exit Play mode (Size: 2Mb)
7. Change The "Sprite Packer" "Mode" from "Always Enabled" to "Always Enabled(Legacy Sprite Packer)"
8. Enter Play mode
9. Notice "Size: 2Mb"
10. Restart Editor and Enter Play mode (Size: 21Mb)

Expected Result: When the "Always Enabled(Legacy Sprite Packer)" is re-selected, after play mode the sprites are re-packed with the proper settings and size
Actual Result: Sprite Packer doesn't pack the sprites appropriately

Reproduced with: 2020.1.0a1, 2019.3.0b1, 2019.2.3f1, 2019.1.14f1, 2018.4.7f1
Did not reproduce on: 2017.4.32f1 (Size doesn't change, so can't confirm or deny repro)

Note:
The issue appeared after https://fogbugz.unity3d.com/f/cases/1170597/ fix. Edit: seems like this issue was always present