Hi Ville,
This is somewhat expected behaviour at the moment. The ability to pass through "AT-style" or "timespec" was never fully supported, but we could pass those values through to rrdtool where provided.
Some of the graph features require us to generate a "period" from the from/to values to do things like showing the "previous" graphing period. I reworked the code that handled this recently because it was split across multiple files and wasn't handling dates correctly.
There's not really an easy way to do this stuff with AT/timespec in PHP, so we may drop this ability as it will always break certain graph types.
The "official" way to do these sorts of time periods is with period=<graph period in seconds>
I've restored passthrough of AT/timespec from/to variables on graph.php, but beware that they won't behave the same as the other time passing methods!