Calls to Map Numbers in merged project only work in one merged project

I am using RoboHelp Classic 2019 to generate Responsive HTML5 output. I have a master project with 2 merged subprojects. I have map numbers defined in the master project and each of the subprojects. For example:

Master:

     #define Topic_A 10

Subproject 1:

     #define Topic_B 110

Subproject 2:

     #define Topic_C 210

I have the following calls in an HTML page to test opening the links:

<a href="http://localhost/Folder/index.htm?rhmapno=10">Link to Topic A</a>

<a href="http://localhost/Folder/index.htm?rhmapno=110">Link to Topic B</a>

<a href="http://localhost/Folder/index.htm?rhmapno=210">Link to Topic C</a>

However, only the link to Subproject 1 is opens the appropriate topic. The other links open the master project index.htm start page.

I have not renamed BSSCDefault.h in any of the files (though I tried that). I have checked that the csh.js file in master and each subproject shows the appropriate topics and mappings.

I can link directly to the topics using /topic.htm#t=mergedProjects/Folder/Subfolder/Topic_C.htm - but I was hoping to use map numbers instead of direct URLs.

Any suggestions?