Exact URL of where the bug happened

API endpoint: https://lichess.org/api/study/by/cdtest
Relevant study: https://lichess.org/study/jZFKec7S

Steps to reproduce the bug

1. Create a new public study with "enable sync" set to "no".
2. Read study metadata updatedAt from /api/study/by/[username]
3. Add a move to the study
4. Read study metadata updatedAt from /api/study/by/[username] again

Example:

1. I created study /study/jZFKec7S with sync disabled and the moves 1. d4 Nf6.
2. /api/study/by/cdtest returned updatedAt: 1684752387426 for this study.
3. I added the moves 2. Bg5 Ne4
4. /api/study/by/cdtest still returned updatedAt: 1684752387426 for this study.

What did you expect to happen?

In step 4, I expected the updatedAt timestamp to be later than the one in step 2.

What happened instead?

In step 4, the updatedAt timestamp was identical to the one in step 2.

Operating system

Arch Linux 5.18.7-arch1-1 (API access) and Windows 7 (study input)

Browser and version (or alternate access method)

wget 1.21.3 (API access) and Chrome 109.0.5414.120 (study input)

Additional information

Use case: My project keeps a copy of the user's Lichess studies. A current updatedAt response from this API endpoint would let me check for changed moves without fetching the entire PGNs.