I originally reported this against Sway, but it seems that this is possibly a problem with XWayland.
To summarize, if I change the scale of a display in Sway, new windows opened with an existing XWayland instance are overscaled. This also seems to happen on hotplugging events.
There's also additional information in a post to the mailing lit some months ago: https://lists.x.org/archives/xorg-devel/2018-July/057305.html
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