This is in TTY emacs from master (exact commit is below), run in
standard Terminal app under macOS 10.14.5:

emacs -Q -nw

open 2 windows: 

1) (above) some long-running compilation process, put point in the end
so it's constantly scrolled

2) (below) open some buffer (dired, for example) and make Terminal-level
selection, so that the point is inside it

Now, at some point, selection in the lower window becomes garbled,
looking like the line is redrawn, but selection not restored.  This can
be seen in attached screenshot.  This has transient effect, appearing
for a short period of time (few seconds).

Text in the upper window, on the contrary, sometimes becomes "selected",
this is also transient.

Thanks.

In GNU Emacs 27.0.50 (build 17, x86_64-apple-darwin18.6.0, NS appkit-1671.50 Version 10.14.5 (Build 18F132))
 of 2019-06-24 built on fgunbin.local
Repository revision: f3b1b5fb5034de026adc41cf2403cff42f4a0b67
Repository branch: HEAD
System Description:  Mac OS X 10.14.5