If the communication works with unicast but not with broadcast, then the issue is not coming from the ArtSync packet MadMapper sends.
If the unicast table doesn't fill automatically and you have to enter IP addresses manually in MadMapper, it means that the ESP ArtNet library you're using doesn't handle ArtNet Polling. If in MadMapper, "Auto-Poll for devices" is enabled it should fill automatically.
If the broadcast packets from MadMapper are not received while with Resolume they are, it might be related to the timing network packets are sent. MadMapper tries to send all ArtNet packets in a short time to improve synchronization. Maybe when 100 ArtNet packets are sent within a short time, say 0.1ms, the network chipset of the ESP32 looses packets ? I have never done this test so I can't tell you.
Can you do the following tests:
1- Does broadcasting from MadMapper work if sending just one universe ? If yes, add universes one by one and see when it stops working
2- What happens if you reduce the ArtNet sending FPS in MadMapper preferences ?
Question: macOS or Windows ?