We can use byteman to address the issue of the additional "[SLA]..." timer:

1. Download and unzip byteman [1]

2. Create a file FixSlaTimerIssueWithLegacyProcesses.btm with the following content:

RULE Fix slatimer node
CLASS org.jbpm.marshalling.impl.JBPMMessages$ProcessInstance$NodeInstance
METHOD getSlaTimerId
AT EXIT
BIND slaTimerId:long = $!
IF (slaTimerId == 0)
DO

1. System.out.println("[BYTEMAN] overwriting node.getSlaTimerId return -1 instead of: " + slaTimerId + " for node instance " + $0);
   return -1;
   ENDRULE

RULE Fix HT node errorHandlingProcessInstanceId
CLASS org.jbpm.marshalling.impl.JBPMMessages$ProcessInstance$NodeInstanceContent$HumanTaskNode
METHOD getErrorHandlingProcessInstanceId
AT EXIT
BIND errorHandlingProcessInstanceId:long = $!
IF (errorHandlingProcessInstanceId == 0)
DO

1. System.out.println("[BYTEMAN] overwriting HumanTaskNode.getErrorHandlingProcessInstanceId return -1 instead of: " + errorHandlingProcessInstanceId + " for work item " + $0.getWorkItemId());
   return -1;
   ENDRULE  

3. Edit $RHPAM_HOME/bin/standalone.conf

if [ "x$JAVA_OPTS" = "x" ]; then
JAVA_OPTS="-Xms1303m -Xmx1303m -XX:MetaspaceSize=96M -XX:MaxMetaspaceSize=1024m -Djava.net.preferIPv4Stack=true"
JAVA_OPTS="$JAVA_OPTS -Djboss.modules.system.pkgs=$JBOSS_MODULES_SYSTEM_PKGS -Djava.awt.headless=true"

1. enable byteman to address timer issues with legacy process instances
   BYTEMAN_HOME="/data/jboss/work/byteman-download-4.0.18"
   SCRIPT_HOME="/home/mweiler/support/03200412"
   SCRIPT_NAME="FixSlaTimerIssueWithLegacyProcesses.btm"
   JAVA_OPTS="$JAVA_OPTS -javaagent:$BYTEMAN_HOME/lib/byteman.jar=script:$SCRIPT_HOME/$SCRIPT_NAME -Dorg.jboss.byteman.verbose=true"
   else
   echo "JAVA_OPTS already set in environment; overriding default settings with values: $JAVA_OPTS"
   fi

4. The /timers REST call should no longer be returning the empty [SLA]... timer

[1]
https://byteman.jboss.org/downloads.html

1. In mysql, create a database 'bpms64kie':
$ mysql -u root -p -e "create database bpms64kie";

2. Import the dump:
$ mysql -u root -p bpms64kie < bpms64kie_db.dump

3. Grant access to user jboss/jboss:
$ mysql -u root -p -e "grant all privileges on bpms64kie.* to 'jboss'@'localhost'";

4. Configure the RHPAM 7.x instance:
<property name="org.kie.server.persistence.ds" value="java:jboss/datasources/mysqlpamDS"/>
<property name="org.kie.server.persistence.dialect" value="org.hibernate.dialect.MySQL57InnoDBDialect"/>
<property name="hibernate.hbm2ddl.auto" value="none"/>

<xa-datasource jndi-name="java:jboss/datasources/mysqlpamDS" pool-name="mysqlpamDS">
<xa-datasource-property name="URL">
jdbc:mysql://localhost:3306/bpms64kie
</xa-datasource-property>
<driver>mysql</driver>
<security>
<user-name>jboss</user-name>
<password>jboss</password>
</security>
<xa-pool>
<min-pool-size>10</min-pool-size>
<max-pool-size>100</max-pool-size>
</xa-pool>
<validation>
<valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLValidConnectionChecker"/>
<validate-on-match>true</validate-on-match>
<background-validation>false</background-validation>
<exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLExceptionSorter"/>
</validation>
</xa-datasource>
<xa-datasource jndi-name="java:jboss/datasources/ejb_timer" pool-name="ejb_timer">
<xa-datasource-property name="URL">
jdbc:mysql://localhost:3306/bpms64kie
</xa-datasource-property>
<driver>mysql</driver>
<security>
<user-name>jboss</user-name>
<password>jboss</password>
</security>
<xa-pool>
<min-pool-size>10</min-pool-size>
<max-pool-size>100</max-pool-size>
</xa-pool>
<validation>
<valid-connection-checker class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLValidConnectionChecker"/>
<validate-on-match>true</validate-on-match>
<background-validation>false</background-validation>
<exception-sorter class-name="org.jboss.jca.adapters.jdbc.extensions.mysql.MySQLExceptionSorter"/>
</validation>
</xa-datasource>

5. Start the RHPAM instance / kie-server

6. Run the script:
$ sh
case03200412.sh
true
...
Process 9 created for process case03200412.timertest
...
> GET /kie-server/services/rest/server/admin/containers/example:case03200412:1.0/processes/instances/9/timers HTTP/1.1
{
"timer-instance" : [ {
"name" : "do something-",
"id" : 1,
"timer-id" : 1,
"activation-time" :

,
"last-fire-time" : null,
"next-fire-time" : {
"java.util.Date" : 1665424800000
},
"delay" : 13292725434,
"period" : 0,
"repeat-limit" : -1,
"process-instance-id" : 9,
"session-id" : 20
} ]

// compare this to the instance 6, which has been created in BPMS 6.4
> GET /kie-server/services/rest/server/admin/containers/example:case03200412:1.0/processes/instances/6/timers HTTP/1.1
{
"timer-instance" : [

, {
"name" : "",
"id" : 1,
"timer-id" : 1,
"activation-time" :

,
"last-fire-time" : null,
"next-fire-time" : {
"java.util.Date" : 1665424800000
},
"delay" : 13295169263,
"period" : 0,
"repeat-limit" : 0,
"process-instance-id" : 6,
"session-id" : 15
} ]