It looks like my guess was correct.
The following is an excerpt from a financial blog post which I found to be a comprehensive, yet understandable summary of all the changes under TCJA:
Capital Gains And Qualified Dividends Retain Old Thresholds Under TCJA
Under current (soon-to-be-prior) law, the thresholds for the 0%, 15%, and 20% long-term capital gains (and qualified dividend) rates are based on the thresholds for the individual tax brackets: those who fall in the 10% and 15% ordinary income brackets get 0% rates, while income in the 25%, 28%, 33%, or 35% brackets gets the 15% capital gains rate, and income in the top 39.6% bracket gets the 20% preferential rate. (In addition, the 3.8% Medicare surtax on Net Investment Income applies on top, producing a maximum capital gains rate of 23.8%.)
However, while the new TCJA rules introduce new tax brackets, and slightly re-draw the tax bracket thresholds, preferential rates for long-term capital gains and qualified dividends will continue to use the old thresholds. As a result, preferential capital gains and qualified dividend rates will no longer line up cleanly with the ordinary income tax brackets.
Instead, the 0% capital gains rate will end at $38,600 for individuals (and $77,200 for married couples), even though the bottom two tax brackets end at $38,700 and $77,400 (although it’s possible a future Technical Corrections act will re-align these).
More significantly, though, the transition from 15% to 20% capital gains rates will also continue to use the “old” top tax bracket thresholds of $425,800 for individuals and $479,000 for married couples – which would now fall in the middle of the new 35% brackets, rather than being aligned to the top 37% brackets. Even as the 3.8% Medicare surtax on net investment income will also continue to apply with its own not-indexed-for-inflation thresholds of $200,000 of AGI for individuals ($250,000 for married couples).
For those interested the link is: