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.