Indexing Service setting becomes enabled after each boot

After every day boot, I check the properties of a temporary 800 MB ramdisk and see that the check box "Allow Indexing Service to index this disk for fast file searching" is ticked, this slows the ramdisk.
However, the Windows XP SP3 indexing service is disabled. I used Softperfect Ramdisk v 3.4.3 and now v 3.4.6, the problem is present with both versions.

If I remove the tick mark on the check box (every day), why the software does not remember the next day that I do not want to use the Windows Indexing Service on this temporary ramdrive?

That's a Windows setting, not a RAM disk's one. When you tick this option, a corresponding attribute is set in the file system, however upon restart the RAM Disk's file system is re-created from scratch effectively losing this bit.

So this behaviour is by design. Setting this option has no effect anyway when the indexing service is disabled.

It seems I found a solution.
Yesterday I unmounted/unistalled the 824 MB temporary ram disk then created it again leaving blank the volume name, so the volume name is not the same as that of the previous ram disk.
Today I turned on the PC and the box "Allow the Windows Indexing Service to..." is not ticked.
Now, the ramdisk is behaving OK though I do not know why this solution worked.
Cheers!

Another option might be setting a relevant Group Policy to disable removable disks indexing.

This can be done in gpedit.msc at Computer Configuration - Administrative Templates - Windows Components - Search. Change Do not allow locations on removable drives to be added to libraries as described in this article: Prevent System Volume Information folder creation on USB in Windows 8.1.