Message Title

			<plugin>
				<groupId>org.hibernate.orm.tooling</groupId>
				<artifactId>hibernate-enhance-maven-plugin</artifactId>
				<version>${hibernate.version}</version>
				<executions>
					<execution>
						<configuration>
							<failOnError>true</failOnError>
							<enableLazyInitialization>true</enableLazyInitialization>
							<enableDirtyTracking>true</enableDirtyTracking>
							<enableAssociationManagement>true</enableAssociationManagement>
							<enableExtendedEnhancement>false</enableExtendedEnhancement>
						</configuration>
						<goals>
							<goal>enhance</goal>
						</goals>
					</execution>
				</executions>
			</plugin>

If you also change the parent pom version to spring boot 2.2.0.M4 you will get Hibernate 5.4. I also tried with 5.4.3.Final by manually setting hibernate.version. In all cases you get a java.lang.VerifyError at runtime (e.g. tests all fail). Example:

java.lang.VerifyError: 
Bad type on operand stack
Exception Details:
  Location:
    org/springframework/samples/petclinic/owner/Pet.$$_hibernate_write_visits(Ljava/util/Set;)V @319: invokevirtual
  Reason:
    Type 'org/springframework/samples/petclinic/owner/Pet' (current frame, stack[1]) is not assignable to 'java/lang/Integer'
  Current Frame:
    bci: @319
    flags: { }
    locals: { 'org/springframework/samples/petclinic/owner/Pet', 'java/util/Set', '[Z', '[Ljava/lang/Object;', integer }
    stack: { 'org/springframework/samples/petclinic/visit/Visit', 'org/springframework/samples/petclinic/owner/Pet' }
  Bytecode:
    0x0000000: b801 474d 2ab4 0004 c700 0b2c 104f 0454
    0x0000010: a700 7c2a b400 0413 0125 b801 179a 000b
    0x0000020: 2c10 5004 54a7 0067 2ab4 0004 b901 2901
    0x0000030: 004e 0336 042c 1051 0454 1504 2dbe a100
    0x0000040: 0b2c 1052 0454 a700 462b c700 0b2c 1053
    0x0000050: 0454 a700 1d2b 2d15 0432 b901 2202 0099
    0x0000060: 000b 2c10 5404 54a7 001a 2c10 5504 542d
    0x0000070: 1504 3201 57c0 0074 01b6 012e 2c10 5604
    0x0000080: 5484 0401 2c10 5704 54a7 ffb1 a700 032a
    0x0000090: 4e2b 3a04 2db6 00f5 c600 252d 2db6 00f5
    0x00000a0: 2d13 0124 2db4 0004 1904 b901 0a05 00c0
    0x00000b0: 0071 b500 042c 1058 0454 a700 142d 1904
    0x00000c0: c000 71b5 0004 2c10 5904 54a7 0003 2bc7
    0x00000d0: 000b 2c10 5a04 54a7 007b 2b13 0125 b801
    0x00000e0: 179a 000b 2c10 5b04 54a7 0069 2bb9 0129
    0x00000f0: 0100 4e03 3604 2c10 5c04 5415 042d bea1
    0x0000100: 000b 2c10 5d04 54a7 004b 2d15 0432 1301
    0x0000110: 25b8 0117 9a00 0b2c 105e 0454 a700 2b2d
    0x0000120: 1504 32c0 0074 b601 312a a600 0b2c 105f
    0x0000130: 0454 a700 152d 1504 322a 57c0 0074 2ab6
    0x0000140: 012e 2c10 6004 5484 0401 2c10 6104 54a7
    0x0000150: ffac a700 032c 1062 0454 b1            
  Stackmap Table:
    append_frame(@19,Object[#329])
    same_frame(@40)
    append_frame(@58,Object[#299],Integer)
    same_frame(@73)
    same_frame(@85)
    same_frame(@106)
    same_frame(@111)
    same_frame(@129)
    chop_frame(@140,2)
    same_frame(@143)
    append_frame(@148,Object[#21],Object[#113])
    same_frame(@189)
    chop_frame(@206,2)
    same_frame(@218)
    same_frame(@236)
    append_frame(@251,Object[#299],Integer)
    same_frame(@266)
    same_frame(@287)
    same_frame(@309)
    same_frame(@327)
    chop_frame(@338,2)
    same_frame(@341)

	at org.springframework.samples.petclinic.owner.VisitControllerTests.init(VisitControllerTests.java:61)

I found I can "fix" it by removing the <enableAssociationManagement>true</enableAssociationManagement>, so I guess that's a workaround.