How to reproduce:
1. Open the attached (Repro.zip) project
2. Open "defaultSample" Scene -> expand Canvas -> Select "AeConvert" GameObject
3. Open the Timeline Window -> move the Timeline marker on the start of Footage_B to activate it
4. Add keys on the Timeline Curve (if all added keys appear, then Activate the Footage_A Animation and add keys there)

Expected result: All keys appear in the Timeline Animation Track Curve when adding keys
Actual result: Not all keys appear in the Timeline Animation Track Curve when adding keys

Reproducible with:2019.3.1f1[1.2.9], 2020.1.0a23 [preview.7 - 1.3.0]
Not reproducible with: 2019.3.0f1(1.2.7)

Notes:
- Could not test on 2018.4, because Timeline was still built-in in that version