I might have an answer… I was setting VSCode up so I could run the stable and development versions of PlatformIO side by side, and was looking at the documentation for the VSCode plugin since Ivan pointed me to it… and came across this setting…
platformio-ide.reopenSerialMonitorDelay
Go into your VSCode settings …
… and put it in (2 seconds = 2000 milliseconds, or whatever delay you think you need), adding a comma to the end of the line above if needed (only omit it on the last entry) …
And let me know how you go?