The subscription isn't being paid because the MD5 check is probably failing. The MD5 was going to be discontinued, we were preparing for this, but they broke the MD5 before it was discontinued. Then they decided to extend usage of MD5, but stopped sending the MD5, but only sometimes. It's a complete disaster on Authorize.net part and their forums are exploding with people having integrations now fail. 8 years of stability gone in a rather short window with no means of working around it except to shut off security checks, which is obviously terrible to have to do but is what everyone else is basically doing and looks like what we'll be having to also do.
Typically API is versioned to prevent backwards compatibility issues like this with APIs. Stripe and PayPal both version their API and I highly recommend either over using Authorize.net.
And our MD5 hash has always been empty. So I can't see how either of these pertain to us.
The changes they've made are impacting single and recurring payments from reports here and on Authorize.net forums. We're still investigating and don't have a solid solution to resolve all the issues yet.