Once saved — always saved has never made biblical sense either.
Man has a free will and can turn on God at any time.
It is easy to not understand OSAS, if one is dichotomous in their anthropology, vice trichotomous.
Falling out of fellowship or grieving the Holy Spirit, is a consequence of a mental state effecting our relationship with Him, but He doesn’t cease indwelling our spirit when we are out of fellowship.
Too many people confuse forgiveness with salvation. When we are saved, we are also immediately forgiven, but as soon as we sin again, until we face Him and confess our sins, we are not forgiven them. This doesn’t kill the regenerated spirit, rather we are simply out of fellowship. If we were so killed a second time, then nobody would have salvation, because 1) everybody sins, and 2) everybody returns into fellowship by 1st John 1:9, and 3) there was only one sacrifice for sin and if it were no longer sufficient for salvation, then there is no other to provide for us returning to Him.