I say at times, as it doesn't happen permanently.
Not sure whose program fault it is, P3D or ACE.
For that reason I have backed up the last good aircraft.cfg and thus it goes on.
In attachment shows how aircraft.cfg becomes to look like, when this happens, resulting to disappear the 737 from sim's aircraft menu.

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I don't know why it happens, but deleting the 3 weird characters in front of [fltsim.0] (see attached image) should allow the aircraft to become visible in P3D.

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Thanks for your reply.
What you suggest is not enough, as more than half contents of aircraft.cfg get disappeared permanently, therefore I use the last good file.

Originally I had suspected Windows Defender and MS Smartscreen, but it happened even though they had both been disabled.

What happens if you do a reinstall/repair before starting P3Dv5.2, then selecting the 737-300? Same thing?

Where is P3Dv5.2 installed?

I did re-install 2-3 times.
P3D directory : C:\P3D.

Following some tests, the issue seems to be between CS737 and P3D, that is, every now then P3D (?) destroys the aircraft.cfg file.
I think CS Team should investigate it. Any debugging file will be provided upon CS Team request.

I found a solution adding an entry (whatever) preceded by // on top of aircraft.cfg entries.
Just in case someone else faces this inconvenience....