While gvim is running, mintty displays Unicode UTF-8 characters as question marks

When I fire up gvim in the background in a Cygwin mintty terminal, Unicode UTF-8 characters start getting displayed as question marks. When I quit gvim, mintty reverts to the correct behaviour. In addition, the bold parts of man page output rendered as color also disappear when gvim is invoked in the background. Running reset doesn’t fix the problem. Any ideas on how to fix this?

$ echo -e "xcexbbx0a"
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$ '/cygdrive/c/Program Files (x86)/vim/vim82/gvim' &
[1] 8340
$ echo -e "xcexbbx0a"
?

$ wait # Quit gvim
[1]+  Done                    '/cygdrive/c/Program Files (x86)/vim/vim82/gvim'
$ echo -e "xcexbbx0a"
λ

$ locale
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC="en_US.UTF-8"
LC_TIME="en_US.UTF-8"
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_ALL=
$ uname -a
CYGWIN_NT-10.0 dell 3.1.5(0.340/5/3) 2020-06-01 08:59 x86_64 Cygwin
$ gvim --version # Opens dialog window showing:
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Dec 15 2019 23:09:16)
MS-Windows 32-bit GUI version with OLE support
Included patches: 1-12
Compiled by appveyor@APPVYR-WIN

$ echo -e "xcexbbx0a"
λ

$ (gvim &)
$ echo -e "xcexbbx0a"
λ