Opening Serial Monitor disconnects Port

Arduino IDE 1.8.5 ARM, RasPi-3
Problem is with a FTD1232 USB<->TTL
esp8266-01 ( *** problem does not happen with a NodeMCU ***)
Plug everything in, power up
Open Serial Monitor
Tools, Port, ttyUSB0
Serial Monitor goes away
Open Serial Monitor
Check port - blank - select, now USB1 is avail., select SM goes away
Upload
running, open SM - nothing - Serial.print no output
and on a terminal running "watch -n 1 'ls -l /dev/ttyUSB*' " - Upload again
shows USB0/1 there, not there, now 0, gone, now 0, gone, now 1, now 0 while uploading

Seems I'm not the only one:
https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=205119&p=1271796#p1271796

Reproduce with Arduino IDE 1.8.5 ARM on a Raspberry Pi using the FTDI USB-to-TTL adapter and ESP8266-01, while watching /dev/ttyUSB*. Start by checking the Serial Monitor and Tools > Port behavior during upload. Done means the selected port remains available and Serial Monitor receives Serial.print output.

Written by the indexing model from the issue text.