Encoding problems after update MM 3.3.7 -> 3.3.8

I’ve just updated (via bundle update) MM 3.3.7 to 3.3.8 (along with some other gems) and found a problem with encoding of UTF-8 characters in Sass-compiled CSS files and Uglified JS files. Extended UTF-8 characters are spit out as 2 ISO-Latin-1 letters. Before update all was OK. My CSS files start with @charset "utf-8";. JS files don’t declare encoding (AFAIK it’s not possible?).

Since the problem occurs both in JS and CSS files then I suspect that this is neither Sass nor Uglifier related problem, but maybe Sprockets problem? Anybody had this issue recently? UTF-8 characters get garbled in comments (both in CSS and JS) in a compiled site (middleman build). I use Sass in a normal way (all CSS assets have Sass sources) and Uglifier in atypical way, I guess. I just minify my custom JS without touching Bootstrap and jQuery sources.

Otherwise the compiled CSS and JS is fine, no changes other than garbled UTF-8 text in comments (I’m using SVN to verify changes before/after bundle update / middleman build).

My bundle update changed the following:
activesupport (4.1.8) → 4.1.9
autoprefixer-rails (4.0.2) → 5.1.5
bootstrap-sass (3.3.1.0) → 3.3.3
compass (1.0.1) → 1.0.3
compass-core (1.0.1) → 1.0.3
middleman (3.3.7) → 3.3.8
middleman-autoprefixer (2.3.0) → 2.4.3
middleman-core (3.3.7) → 3.3.8
middleman-sprockets (3.3.10) → 3.4.1
sass (3.4.9) → 3.4.12
sprockets-sass (1.2.0) → 1.3.1
uglifier (2.5.3) → 2.7.0

and probably unrelated:
chunky_png (1.3.3) → 1.3.4
coffee-script-source (1.8.0) → 1.9.0
eventmachine (1.0.3) → 1.0.7
execjs (2.2.2) → 2.3.0
i18n (0.6.11) → 0.7.0
jquery-cdn (1.11.1) → 1.11.2
json (1.8.1) → 1.8.2
listen (2.8.3) → 2.8.5
minitest (5.4.3) → 5.5.1
net-ssh (2.9.1) → 2.9.2
ptools (1.2.7) → 1.3.2
rack (1.5.2) → 1.6.0
rack-test (0.6.2) → 0.6.3
uber (0.0.11) → 0.0.13