I think its more a matter of different fronts he puts on for different situations. Sarek could appear very different depending on occasion, with more formality and stiffness in "Journey to Babel" to honest and pained in Star Trek III, willingly acknowledging his illogic.It doesn't make narrative sense that Frain hasn't become Lenard Sarek yet. We've already seen that Sarek in "Yesteryear" and Star Trek V prior to Discovery. He was not the warm, fuzzy smiling version Frain plays. He should already be the Sarek we know and love from TOS and TNG.
The fact that he remains "the same" through TNG is more off putting to me than Frain's portrayal.