As a Ruffle developer who in my day job maintains some Flash-based websites, I'll note from experience that AMF serialization/deserialization in Ruffle has some definite issues, so that may be the issue for your games (the websites I maintain use https://metacpan.org/pod/AMF::Perl). See https://github.com/ruffle-rs/ruffle/issues?q=is:issue+state:....
As far as I've seen, Ruffle never even makes the call out to the server... so at this point I don't think it's a serialization issue although some of what's in that list could potentially cause problems. The Ruffle compatability docs still say that NetConnection has 90% coverage... except for the .connect() call itself, which kinda makes me wonder why bother covering it at all?
https://ruffle.rs/compatibility/avm2
That documentation, for stubs, can be somewhat misleading. It just looks for the presence of an avm2_stub_method function call anywhere in the method, which may mean a method that's entirely a stub, or as is the case for NetConnection.connect, a method that is stubbed under specific conditions. NetConnection.connect is stubbed for specifically non-null, non-http commands (generally this is RMTP/RTMFP). See https://github.com/ruffle-rs/ruffle/blob/df11c2206bc6be0a329...
I am jolted, nearly shocked, that in 2026 you have to maintain some Flash-based websites. Can you share?
I mean I could decommission them but they're educational websites related to DNA and bioinformatics with interactive animations and my boss has a certain fondness for keeping them running if we can, as we used to get a number of grants that funded creating them in the first place as a nonprofit educational and research institution.