If you've already conditioned it, and it actually tasted where you liked it, then went down hill, more than likely it's an infection developing. But who knows. If you can spare the bottles walk away for a few more months and revisit them. You know it's a beer that is traditionally aged for nearly a year, so it's not a surprise, that you need to sit it for awhile...so it won't hurt to see what becomes of it.
You may be developing some nice infection issues that will lend character to the beer down the line, like Brett and stuff, so it may turn out to have some nice sour or fruit notes down the line. It may surprise you, or it may be a dumper.
But either way, you need to look at potential infection sites, like your hoses, bottling setup, etc, as well as your santization processes, so you don't recreate it in other beers.