You found the bbb (batch beginner bug), but not the variable is empty, it’s the expansion that doesn’t work as expected.
Percent expansion is done when a line or a complete parenthesis block is parsed, before the code will be executed.
But to solve this you can use the delayed expansion, this doesn’t expand at parse time, it expands just at execution time.
setlocal EnableDelayedExpansion
if exist "%_REALPATH%\tomcat-%TOMCAT_VER2%" (
set CATALINA_HOME=%_REALPATH%\tomcat-%TOMCAT_VER2%
set TOMCAT_VER=%TOMCAT_VER2%
echo "!TOMCAT_VER!"
) else if exist "%TOMCAT_VER2%" (
set CATALINA_HOME="%TOMCAT_VER2%"
set TOMCAT_VER="%TOMCAT_VER2%"
echo "!TOMCAT_VER!"
)