Passkeys don't work in Safari web app

I created a safari web app for online accounting software that I use.

I have passkeys configured for this website in Safari.

Until recently I was able to use the passkey from within the web app.

Now I can only use the passkey if I open Safari and navigate to the website. When I try to use the passkey from within the web app it just does not respond.

What I tried:

• Removing the web app, and reinstalling
• Removing the passkey and reinstalling
• Clearing cookies and other website data
• Removing all browser extensions

Unfortunately I do not have other passkeys to test this with.

Are passkeys (still) supported in Safari web apps?