Table cells with MathML content are not sized appropriately, so the content overlaps neighbouring cells, or disappears entirely.
See the testcase.
There is no test case for this bug. Can you upload an example?
A quick test of this looks like https://bugs.webkit.org/show_bug.cgi?id=40986 may fix it.
If you have an example with the problem, I can confirm this.
Created attachment 59414 [details]
Damn! What happened to the testcase I uploaded?
I think this is close to the original (hastily-reconstructed).
It looks like this is fixed by the patch in 40986.
Indeed, the test case seems to be rendered properly now.
The bug status should be changed to CLOSED.