Substack announced Notes on Wednesday, explaining that users “will be able to post short-form content” on “almost anything—including posts, quotes, comments, images, and links.”

Shortly after, Twitter began hamstringing Substack. Now, Twitter blocks likes, retweets, and comments on tweets that have a link to a Substack newsletter. And Substack writers can no longer embed tweets when creating a post on Substack. The issue seems to be only affecting Substack and no other platforms.

Substack CEO and cofounder Chris Best tweeted Friday, “We hope this action was made in error and is only temporary, because writers deserve the freedom to share whatever links they want. But this is bigger than Twitter. It shows why it’s so important for writers to own their relationship with their audience.” 

Twitter did not reply to Fortune’s request for comment. Although it did not give a reason for its moves against Substack, several online critics remarked on the timing.

Theo Priestley, author of the 2021 book The Future Starts Now, tweeted that Twitter was “most likely” suppressing Substack because of the Notes announcement. “Twitter doesn’t want you to use it,” he wrote.

“A day after [Substack] announced its own Twitter-like service called Notes, Twitter seems to now be blocking the ability to embed tweets within Substack posts,” wrote conservative pundit Parker Molloy. “Quite literally the worst thing about Twitter these days (in my opinion) is that things change on a total whim. Every day, we open the site to find something new that has changed for no clear reason.”

Substack differs from social media giants in that it focuses on subscriptions rather than advertising for revenue. Newsletter writers themselves decide how much to charge for subscriptions, and how widely to make their work available. 

“The ultimate goal on this platform is to convert casual readers into paying subscribers,” Substack wrote in its Notes announcement. “In this system, the vast majority of the financial rewards go to the creators of the content.”

Substack issued a statement Friday in which it said it was “disappointed that Twitter has chosen to restrict writers’ ability to share their work…their livelihoods should not be tied to platforms where they don’t own their relationship with their audience, and where the rules can change on a whim.” 

Whimsical changes have been commonplace on Twitter since Musk’s $44 billion takeover last year. Last weekend, Twitter removed the blue check mark from the New York Times account on the platform in the start of a purge of “legacy blue checks.” Before Musk, the marks were free and served to verify the identity of notable accounts; now the company wants users to pay a monthly fee for them. 

In December, the company suddenly forbade users from linking out to competing social networks, only to quickly reverse that decision. Paul Graham, a widely respected venture capitalist who’s been supportive of Musk, had his account briefly suspended because of the policy change after he mentioned his account on Mastodon.

Musk tweeted in December: “Twitter should be easy to use, but no more relentless free advertising of competitors. No traditional publisher allows this and neither will Twitter.”

The new Notes service will not likely pose much of a threat to Twitter or other established social media giants. As Substack wrote in its announcement: “The incumbents are entrenched.” 

But on Friday afternoon, Substack tweeted: “Any platform that benefits from writers’ and creators’ work but doesn’t give them control over their relationships will inevitably wonder how to respond to the platforms that do.”