Thanks @ptillisch for the reply.
FYI, my sketch already uses WiFiS3.h.
I loaded WiFi101_OTA on my board and the OTA network port seems more stable. So it’s probably like you said:
When not connected to the USB port on my PC, I use a 30W (5V-3A) power supply during testing or a 70A 12V regulated power supply during operation. So, I don’t think the power supply is the issue.
As soon as I load my sketch, the OTA network port disappears.
After commenting the MQTT connection part of my code, the OTA network port stabilizes.
In my test environment, the MQTT broker is not reachable, so I’ll try tomorrow in the production environment to see if it fixes the issue.
I’ll post my results.