In markdown cells, replacing a string with text that contains the string fails

[JasonWeill]

Description

This is a follow up to #16242, itself a follow up to #15572.

In a markdown cell, replacing a string with a replacement string that contains the search string at least once (for example, tom replaced with tommy or tomtom) does not cause the highlight to go to the next cell.

Reproduce

1. Create two markdown cells, each containing the word tom twice.
2. Search for tom and replace it with a string that contains tom at least once.
3. Replace the first two instances in the first cell.
4. The highlight does not move to the second cell.

Expected behavior

The first match in the second cell is highlighted.

[JasonWeill]

I do not see this same behavior with 'raw' cells.