Replace is not working for a marked text region. Steps to repro,
Type in the following
11 22 33 44 55 66 77 88 99
select the above text. Open the Replace dialog and replace the " " with ",0x". The result is the text is only replaced up to the originally marked text, but since the text grows in length, the items that fall out of the marked region do not get replaced.
Also, consider if there is a marked text region, when Replace dialog box appears, the Marked Text only region should default.
Replace in Marked Text Region
Thanks for the bug report and the clear steps on how to replicate itReplace is not working for a marked text region. Steps to repro,

This is how it is meant to workAlso, consider if there is a marked text region, when Replace dialog box appears, the Marked Text only region should default.

The marked region option will only get selected if more than one line of text is selected or if the whole line is selected in line marking mode.
The reason it works this way is because for the case where less than a whole line of text is marked, the marked text is used to populate the find text field.
Cheers Jussi