Feeling kind of dumb, but I did figure it out so I thought it was worth sharing my embarrassment in case it helps somebody else...
Following the news that Mobile Firefox is going to start allowing desktop plugins(!)
https://www.androidpolice.com/firefox-beta-107-android-install-any-browser-add-on/
I finally got off my butt and installed Firefox as my default browser.
Buuuut... Voyager was still opening links in Chrome, even though Firefox is the new default. Restarted the phone, no dice, then it hit me...
"Dummy, you installed the web app in Chrome..."
Uninstalled the web app, re-installed from Firefox, now all the links open in Firefox.
Admit your mistakes, learn from them, share them with others. ;)
Ymmv, but I haven't had a smooth experience with WebApps installed with firefox - I've only tired voyager and freshrss, but there seems to be input latency or something. I don't have the issue with firefox in general, just with the "installed" web apps.
I ended up having a much better experience with installed WebApps via Vanadium instead.
I've had the same experience. I use voyager from the Firefox browser directly and didn't install the app.