Marvel made Transformer comics across two offices in New York and London for 10 years (1984-1994) without ever rebooting continuity. There were a few retcons made but never a whole continuity reboot. Continuity became more consistent after 1989 when Furman replaced Budiansky in writing the US comics allowing for greater fidelity between the US and UK stories.

I don't think we really had a 'real' continuity reboot until 2003 with Transformers Armada (or arguably 2000's Car Robot (RiD), but it's since been officially retconned as being in-continuity with G1, so technically Armada is the first continuity reboot that remains a reboot).