Thanks for your report. Though I cannot reproduce the problem.
Please start WinSCP with /applog=C:\some\path\app.log command line parameter, enable also session logging, reproduce the problem, and post both app and session logs.

Using WinSCP 6.5.5 with "Explorer" interface on Windows 11 Enterprise 23H2.

I have two remote sessions open, and I'm using NP++ to edit a text file from HostB. If WinSCP has the tab from HostA selected and I save a change to the file on HostB, I get the following message:

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Confirm
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The remote file has been changed while you were editing it. Do you want to overwrite it anyway?
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OK Cancel Help
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This happens every time. If the tab for HostB is selected, there's no message when I save the file. Not a serious issue that I can see, but it is bothersome.

This is a typical scenario for me (editing a file on one remote host while I'm looking at other remote hosts), so I have to say that it started happening with v6.5.5.

Thanks.