can you try safe mode? nevermind, the table builder is a theme component. 
i cannot reproduce this on my forum - i just edited a table in Safari and Chrome.
here is a test table on Meta
| Column 1 | Column 2 | Column 3 | Column 4 |
|---|---|---|---|
| 1 | A | x | hello |
| 2 | B | y | world |
| 3 | C | z | meow |
| 4 | D | az | woof |
| 5 | E | bz | tweet |
also successfully edited the above table here on Meta
theme component is up to date too. hmmm maybe trying re-installing it? also make sure your Discourse instance is up to date if you haven’t already.