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.