UART not reliable on legato 19.07

We are working on new project and we were testing our app on mangOH Red with processor WP8548 and legato 18.6.3. We compiled the project for legato 19.07 and uploaded it to mangOH yellow. We have noticed, we have got massive problem with UART on mangOH yellow. The code which was working on mangOH red without problems has become unreliable on mangOH yellow.
Symptoms:
When we send 5-8 bytes to mangOH yellow via UART1 ( IoT slot ) event UartHandler fires but no data is read (read function returns 0 bytes ‘No Data’ straight away )

void UartHandler(int FD, short Event)
{
	if (Event & POLLIN)
	{
		uint8_t u8RxByte;
		if (read(FD, &u8RxByte, sizeof(u8RxByte)) > 0)
		{
			LE_INFO("Byte read");
		}
		else
		{
			LE_INFO("No Data");
		}
	}
}

When we send small packets ( 5-8 bytes ) of data every 500ms, first packet or first bytes are ignored, but later system works fine and next packets of data are read correctly
When we send small packets ( 5-8 bytes ) of data every 60s usually everything is ignored. The event fires but no data coming to the system.

The problem does not exist on mangOH red as I said before.

I do not know if the problem is related to unit or legato framework, so I have also created a topic on mangOH forum.

I think, it is legato 19.07 problem, not the unit. I have uploaded legato 18.6.1 to mangOH yellow and it is ok now.

UPDATE:
I have come back from legato 18.6.1 to legato version 19.07. When I installed 19.07, UART started to work without issues. Luckily I have got 2 mangOH yellow. First mangOH yellow has got original legato 19.07, second one has got legato 19.07 which I have just reinstalled (from 18.6.1). UART on device with original legato does not work correctly, but works ok on device with reinstalled legato 19.07.
I tried to keep UART awake and I used Power Managment functions:

le_pm_NewWakeupSource
le_pm_StayAwake
and
le_fdMonitor_SetDeferrable

NO EFFECT.

I have got 2 the same units with legato 19.07. One works fine second does not. There are settings somewhere to keep UART awake.

@TheGod,

From where you have got the new legato 19.07?
Can you share the result of “legato version” for both mangoh_yellow please?