Since update my iPad to 14.3 my WIFI disconnects when the iPad goes to sleep. It never did this before.

If your iPad disconnects when idle (sleep/locked) you may need to verify the your WiFi Settings for each of your configured WiFi Networks - ensuring that they are set to Auto Join:

Settings > WiFi > [WiFi Network / SSID] - tap the “i” icon - set Auto Join to ON

Unless Auto Join is enabled, your iPad will not automatically reconnect to the WiFi network when resuming from sleep - or when coming into range of your WiFi network.

Auto Join is enabled/disabled for each configured WiFi network.

When the iPad goes to sleep and is not connected to power, wi-fi has always disconnected temporarily. It is a battery saving feature and is configured to work this way. As soon as you wake the iPad up, the wi-fi should automatically reconnect. It is working as designed.