Tested on Windows XP. Will test it on other platforms and update the CR.
There were no issues during install. This is reproducible.

Machine details, no authentication required:
IP: 10.5.34.140

Tested in below scenarios:
6u21-b05(b507) -> 6u21-b05(b508)
6u21-b05(508) -> 6u22-b508


Steps to reproduce:

1) Clean the system by uninstalling any JRE's on the system.

2) Install 6u21-b05 (b507) JRE.
http://j2se.east.sun.com/arc/1.6.0_21/nightly/2010-05-23/windows-i586/jre-6u21-windows-i586-iftw.exe
  After this step evar3 is set to "XX" in jusched.log - expected.

3) Autoupdate to next build (b508) of same version of JRE.
Set UpdateServletURL and ManualUpdateServletURL registry keys under
[HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Update\Policy] to below map file.
Used the map file from:
http://javaweb.sfbay.sun.com/~ranjitl/deployment/setupfiles/map-1.6.0_21_b508.xml
Launch Java Control panel, select "Update" tab and select "Update Now" to manually
update to next build.
  After this step evar3 is set to "XX" in jusched.log - this is not expected.

4) Autoupdate to next version 1.6.0_22(b508).
Set UpdateServletURL and ManualUpdateServletURL registry keys under
[HKEY_LOCAL_MACHINE\SOFTWARE\JavaSoft\Java Update\Policy] to below map file.
Used the map file from:
http://javaweb.sfbay.sun.com/~ranjitl/deployment/setupfiles/map-1.6.0_22_b508.xml
Launch Java Control panel, select "Update" tab and select "Update Now" to manually
update to next build.
  After this step evar3 is set to "1.6.0_21" in jusched.log - expected.

Attached jusched.log and MSI logs for reference.

This issue is not reproducible on prior versions of JRE.
1)6u15 -> 6u21-b508(http://j2se.east.sun.com/arc/1.6.0_21/nightly/2010-05-23/windows-i586/NEXTBLD/iftw/au-descriptor-1.6.0_21-b508.xml)
evar3 is set to 1.6.0_15 as expected

2)6u20-b71 -> 6u21-b508
evar3 is set to 1.6.0_20 as expected
Few testcases in AU_NextGen testsuite fail due to "XX" ping

AutoUpdate_NextGen/JREUpdate_Manual_X-1ToXToXPlus1
AutoUpdate_NextGen/JREUpdate_Auto_X-1ToXToXPlus1
AutoUpdate_NextGen/Downgrade_AU_JREUpdate
AutoUpdate_NextGen/AUUpdate_UI_Kernel-01
AutoUpdate_NextGen/AUUpdate_Silent_Static_Online