MultiSchemaMultiTenantProcessEngineConfiguration config =
        new MultiSchemaMultiTenantProcessEngineConfiguration(tenantInfoHolder);
config.setDatabaseType(DATABASE_TYPE_MSSQL);
config.setDatabaseSchemaUpdate(DB_SCHEMA_UPDATE_TRUE);
config.setUseLockForDatabaseSchemaUpdate(true);  // required to trigger the bug

ProcessEngine engine = config.buildProcessEngine();

tenantInfoHolder.addTenant("tenant");
config.registerTenant("tenant", hikariFor("my_db"));   // hangs forever

package repro;

import org.flowable.common.engine.impl.cfg.multitenant.TenantInfoHolder;

import java.util.Collection;
import java.util.LinkedHashSet;
import java.util.Set;

public class InMemoryTenantInfoHolder implements TenantInfoHolder {
    private final Set<String> tenants = new LinkedHashSet<>();
    private final ThreadLocal<String> current = new ThreadLocal<>();

    public void addTenant(String tenantId) { tenants.add(tenantId); }

    @Override public Collection<String> getAllTenants()        { return tenants; }
    @Override public String getCurrentTenantId()               { return current.get(); }
    @Override public void setCurrentTenantId(String tenantId)  { current.set(tenantId); }
    @Override public void clearCurrentTenantId()               { current.remove(); }
}

package repro;

import com.zaxxer.hikari.HikariConfig;
import com.zaxxer.hikari.HikariDataSource;
import org.flowable.engine.ProcessEngine;
import org.flowable.engine.impl.cfg.multitenant.MultiSchemaMultiTenantProcessEngineConfiguration;

import javax.sql.DataSource;
import java.util.concurrent.TimeUnit;

public class FlowableRegisterTenantDeadlockRepro {

    public static void main(String[] args) throws Exception {
        InMemoryTenantInfoHolder tenantInfoHolder = new InMemoryTenantInfoHolder();
        MultiSchemaMultiTenantProcessEngineConfiguration config =
                new MultiSchemaMultiTenantProcessEngineConfiguration(tenantInfoHolder);
        config.setDatabaseType(
                MultiSchemaMultiTenantProcessEngineConfiguration.DATABASE_TYPE_MSSQL);
        config.setDatabaseSchemaUpdate(
                MultiSchemaMultiTenantProcessEngineConfiguration.DB_SCHEMA_UPDATE_TRUE);
        config.setDisableIdmEngine(true);
        config.setDisableEventRegistry(true);
        // comment out this line to make the problem disappear
        config.setUseLockForDatabaseSchemaUpdate(true);

        System.out.println("Booting engine ...");
        ProcessEngine engine = config.buildProcessEngine();
        System.out.println("Engine booted.");

        // Post-boot registration: invokes createTenantSchema(...)
        //   -> SchemaOperationsEngineBuild
        //   -> ProcessDbSchemaManager.schemaUpdate
        //   -> LockManagerImpl.waitForLock  ← deadlocks on MSSQL
        tenantInfoHolder.addTenant("tenant");

        Thread registerThread = new Thread(() -> {
            System.out.println("Registering tenant post-boot ...");
            config.registerTenant("tenant", buildHikari("my_db"));
            System.out.println("tenant registered (this line NEVER prints on MSSQL).");
        }, "register-tenant-0");
        registerThread.start();

        registerThread.join(TimeUnit.SECONDS.toMillis(10));
        if (registerThread.isAlive()) {
            System.err.println("\nDEADLOCK CONFIRMED: tenant registration did not complete in 10s.\n");
            for (StackTraceElement e : registerThread.getStackTrace()) {
                System.err.println("  at " + e);
            }
            System.exit(1);
        }
        engine.close();
    }

    private static DataSource buildHikari(String database) {
        HikariConfig cfg = new HikariConfig();
        cfg.setJdbcUrl(
                "jdbc:sqlserver://localhost:1433;databaseName=" + database
                        + ";encrypt=false;trustServerCertificate=true");
        cfg.setUsername("sa");
        cfg.setPassword("yourStrong(!)Password");
        cfg.setMaximumPoolSize(10);
        cfg.setPoolName("DB Connection Pool " + database);
        return new HikariDataSource(cfg);
    }
}