Is this a bug?

I've begun the practice of throwing up my Synastry on the tank, then my HoTs with Diurnal on them(Benefic and Helios), giving them an applicable card, the HoT from Collective Unconsciousness, and then finally Time Dilation to extend it all and DPS'ing as long as possible till all those wear off. Time Dilation extends Synastry on them, but then when it times out on my side, it times out on them too even with the 15+ second difference on each side. So why does the game bother to extend it if it lasts the same amount of time on both sides anyways?

I would say that it's a necessary thing. If you want Celestial Opposition to affect it for its aoe buff extension, Time Dilation has to as well, even if it doesn't really make the buff last longer.

Synastry establishes a link between caster and target. If there's no link between caster and target (it fell off on either side), how would synastry have effect?

The OP understands that I think, he just seems irked as to why it even extends it on the tank's end, but I think it's for simplicity so Celestial Opposition will continue to work as it does.

From the patch notes, we know that time dilation can extend all AST buffs except synastry (for which they cited balance reasons, and it does make sense). It just happens that the code to terminte synastry is set on the AST side instead of the targets'.

I wonder if anyone has used synastry and then removed it intentionally. Maybe it shuts down either way, at which point it is probably working as intended (just that they didn't bother making time dilution check specifically for synastry to send you an error message).

So no, not a bug. But it is just a bit of odd coding.