Max heap value apparently can grow dynamically:
[standalone@localhost:9990 core-service=platform-mbean] cd type=memory
[standalone@localhost:9990 type=memory] ls
heap-memory-usage=

{"init" => 67108864L,"used" => 350389528L,"committed" => 488112128L,"max" => 488112128L}

non-heap-memory-usage=

{"init" => 2555904L,"used" => 300150008L,"committed" => 320233472L,"max" => -1L}

object-name=java.lang:type=Memory
object-pending-finalization-count=0
verbose=false
[standalone@localhost:9990 type=memory] ls
heap-memory-usage=

{"init" => 67108864L,"used" => 369063704L,"committed" => 488112128L,"max" => 488112128L}

non-heap-memory-usage=

{"init" => 2555904L,"used" => 300479104L,"committed" => 320561152L,"max" => -1L}

object-name=java.lang:type=Memory
object-pending-finalization-count=0
verbose=false
[standalone@localhost:9990 type=memory] ls
heap-memory-usage=

{"init" => 67108864L,"used" => 381295200L,"committed" => 489684992L,"max" => 489684992L}

non-heap-memory-usage=

{"init" => 2555904L,"used" => 300878248L,"committed" => 320954368L,"max" => -1L}

object-name=java.lang:type=Memory
object-pending-finalization-count=0
verbose=false
[standalone@localhost:9990 type=memory]

This doesn't seem updated and in the graph you can see committed memory growing above the max value.
Also in the summary box, it may get stuck to 0MB.