The problem would be on the dimmer end.
Maybe the components on the dimmer has ages so that the dimmer timing does not work accurately.
Most probably there is something wrong related to phase A zero cross detection. The zero cross detector detects zero position slightly off what it should (earlier than it should), the dimmer electronics when at full tries to send "turn on" pulse to triacs/thyristors too early: the triacs will not be for sure turned on... then triggered they might start being on for rest of mains phase or might not stay on...
Zero crossing detection on the dimmer is propable cause. Also the mains power filtering and snubber networks on the circuit could affect this operation.