I think you misunderstood.
It's established that the Lich King has a consciousness all of his own, wearing the Helm brings with it an internal mental struggle against this will. Notice that when Arthas first put on the helm, he says "Now, we are one." The entity of the "Lich King" is not just Bolvar. It wasn't just Arthas. The "Lich King" is an amalgamation of consciousness. This internal power struggle is also reflected in Christie Golden's Lich King book, where Ner'zhul and Arthas fight for control. There's a bit of Ner'zhul in there, a bit of Arthas in there, and there's clearly a bit of some other evil presences in there too (which, while never previously explained will doubtlessly be explored in the coming Shadowlands lore).
Bolvar wearing the Helm =/= Bolvar without the helm. When the Helm falls from Arthas' head during the end of ICC cinematic, Arthas' voice changes, and for a moment before death he reverts from "The Lich King" to just "Arthas"
When Bolvar loses the helm to Sylvanas, he reverts from "The Lich King" (the entity we saw in the DK class campaign in Legion, which is an amalgamation of consciousness) to just Bolvar, the noble but tortured soul who chose to become the jailer of the damned and put on the crown in WotLK.
Bolvar himself has probably been affected by his time as the Lich King, but he's not the same person as he is with the helm on, so his voice and personality changing somewhat after the helm is taken from him is not at all unexpected, nor any kind of retcon.
Hopefully that explains what's happening. Any questions, let me know
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No doubt Bolvar has been affected by his time wearing the crown, which might not make him the exact same person now that it's gone, but the difference in voice can be down to a new voice actor in the many years since WotLK, or it could be that the original voice actor is the same but they're older and not quite putting in the same performance (remember when everyone went crazy thinking they had changed Tyrande's voice in Mists and actually it turned out it's the same actress?)