Consider the following MWE:

\starttext
\setupTABLE[
  rulethickness=5pt,
  framecolor=white, %red,
  background=color,
  backgroundcolor=gray,
  frame=off]
\setupTABLE[1][2][foregroundcolor=white, topframe=on]
\setupTABLE[r][2][topframe=on]
\bTABLE[setups=table:color]
  \bTR \bTD One                       \eTD \bTD A \eTD \eTR
  \bTR \bTD \rotate[rotation=90]{Two} \eTD \bTD B \eTD \eTR
\eTABLE
\stoptext

Attached is the result for ConTeXt LMTX version 2021.04.09 (and earlier) and for version 2021.06.18. In the latter case, the white frame is missing in the first column between row 1 and 2. Changing the frame color to red, makes the frame appear.

Could this be a bug or am I doing something wrong?

Thank you for looking into this. Cheers,

Christoph