Is there a grammatical rule to that?
In the two cases with which you began, it is clear to me that a relative clause is needed. But I don't know how one would formulate a general rule.
For comparison, here are some contrary examples that I've found online:
[Photo caption:] Coast Guard members check the still smoldering M/V Mercraft 2 after it was towed to an island. [I did have an example like this in mind.]
When Non-Fungible Tokens (NFT) first came into existence in 2014, they were just another innovation in the still-developing web3 space.
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The local revelers ignore the still youngish tech billionaire sipping a pint in their midst.
So one does come across this kind of usage, perhaps especially in journalistic writing.