PhpStorm showing "unvailable" breakpoint whereas execution is suspended

I'm sometimes stuck while attempting to debug my code. Debug Session is active, code execution is suspended :

But I cannot see what really happens, as the breakpoint show "unavailable" ("no parking" symbol):

Does anybody know about this sign ?

I still haven't found any information about it on JetBrains sites... that's why I'm here :-)

(PhpStorm 2020.3, using docker containers (linux containers) with Docker Desktop/ Windows 10)

[EDIT] : I just noticed that "break at first line in php script" seem to be functioning though:

But I have these weird breakpoints instead of red "normal" ones, and an highlighted line.

I tried restarting my docker containers, same issue. This produces seemingly randomly and gets solved after a while ... (reboot ?...)

[EDIT] SOLVED The path mapping (local<->docker) for the root of my project was empty (how did it happen...) in my docker configuration in PhPStorm. I'm not sure how this problem occured, but I'll be able to solve it next time if it's back.