What will Sierra and Nosler do or did they get their bullets right the first time?
Everyone has known for a long time that Nosler ballistic tips don't have anywhere near the advertised ballistic coefficients. This may be one contributing factor. Nosler publishes BCs as advertising; they have little interest in accuracy.
Sierra, in contrast, has led the industry in BC accuracy for a long time. Whether the TMKs have this issue is an open question. If TMKs were immune from melting, it seems that Sierra would have published this information my now. My guess is that they are looking into it more carefully.
To me, it may be a hint that the TMK has an advertised BC that decreases much faster with velocity (distance?) than the SMK (see figure attached). The melting and flattening of the tip in flight would explain why the TMK's BC decreases faster with distance than the SMK's BC, because the SMK has no tip to melt.