rei's test is useless, for 2 reasons:
1- rei's minimap only have access to client-data, not server data. Hence, if the server dont send mobs to client, rei's minimap cant draw them.
2- server's have a limitation range to send entities to clients. The same way if you are in the nether the server is not sending to your client overworld related data, its the same with entities too far from you. The client beeing unable to see them NOT EQUAL mobs doesnt exists.
And, for the red dots disappearing, if your limit is 128, what happen if enemies spawn at 127 and fall to 129?