And you provided no issue details (capture of SIP session, for example), so no one can analyze the change and give advice. But it seems there's no change at SPA504G side - the Asterisk behavior has changed. So you should ask in an Asterisk supporting discussion/list instead.
I would speculate that someone here runs Asterisk 13 and uses a SPA51X and utilizes call parking. Just curious as to if other are seeing this behavior. And yes I started in the Asterisk forums as well and I thought I would see if anyone here has experienced this issue.