When the following is on a card
<anki-mathjax block="true">\begin{array}{|c|c|c|}
\hline
1 & 2 & 3 \\
\hline
1 & 2 & 3 \\
\hline
\end{array}</anki-mathjax>
it results in a rectangle the opposite colour of the Anki window theme and the size of the what the MathJax should be displaying.
This happens in the editor or card browser, however the correct output is displayed when reviewing the card and on the preview pop-up from the editor pane.
When removing their the top or bottom \hline
the bug stops happening until it is added back, however, the vertical lines around the middle column are not displayed in the editor and browser.
Tested on Anki 2.1.55 beta 6 (ca116699) and Anki 2.1.54 (b6a7760c)