This snippet:

{|
|a
b||c
|}

is parsed as <table><td>a\nb</td><td>c</td></table> by Parsoid and <table><td>a\nb||c</td></table> (simplified normalized HTML) by PHP parser.

This difference in parse affects multiple things (a) serialization of <td>a\nb</td><td>c</td> (b) unnecessary nowiking in <td>a\nb||c</td>.

T149209 is related. The current fix for T149209 doesn't factor in the impact of this edge case difference. As part of fixing this bug, code written for that bug would need to be fixed up as well.