This happens on JRuby 9.3.8.0 on Windows 2022 Datacenter and Java 17.
I can't seem to reproduce on Mac or Linux.

Expected Behavior

It's expected that new StringIO objects will respect the defined Encoding after it changes.

Actual Behavior

puts RUBY_DESCRIPTION
require 'stringio'
puts Encoding.default_external
puts StringIO.new.external_encoding
ENV["test"] = ""
Encoding.default_external = Encoding::UTF_8
puts StringIO.new.external_encoding

PS C:\> .\jruby-9.3.8.0\bin\jruby C:\logstash-8.4.0\test.rb
jruby 9.3.8.0 (2.6.8) 2022-09-13 98d69c9461 OpenJDK 64-Bit Server VM 17.0.4+8 on 17.0.4+8 +jit [x86_64-mswin32]
IBM437
IBM437
IBM437

While on JRuby 9.2 we can see that the change is respected for new StringIO objects:

PS C:\> .\jruby-9.2.21.0\bin\jruby C:\logstash-8.4.0\test.rb
jruby 9.2.21.0 (2.5.8) 2022-06-27 49e5080a7c OpenJDK 64-Bit Server VM 17.0.4+8 on 17.0.4+8 +jit [mswin32-x86_64]
Windows-1252
Windows-1252
UTF-8

This came from minimizing a reproduction in Logstash, and in within Logstash removing forced jit (threshold=0) the issue went away (but would probably return later once the threshold was hit).