User agent not switched for favicon requests

Favicon requests seem to be coming through with an unaltered user agent:

# normal resources are requested with switched user agent

x.x.x.x - - [03/Jun/2018:10:21:45] "GET / HTTP/1.1" 200 564 "-" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:58.0) Gecko/20100101 Firefox/58.0"
x.x.x.x - - [03/Jun/2018:10:21:45] "GET /css/stylesheet.css?ver=2 HTTP/1.1" 200 532 "http://example.com/" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:58.0) Gecko/20100101 Firefox/58.0"

# these favicon requests use my unaltered Firefox user agent

x.x.x.x - - [03/Jun/2018:10:21:45] "GET /images/apple-touch-icon-72x72-precomposed.png?v=3 HTTP/1.1" 200 3990 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Firefox/60.0"
x.x.x.x - - [03/Jun/2018:10:21:45] "GET /images/apple-touch-icon-152x152-precomposed.png?v=3 HTTP/1.1" 200 2522 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Firefox/60.0"

# the favicons are grabbed again but with the switched user agent this time, for some reason

x.x.x.x - - [03/Jun/2018:10:21:45] "GET /images/apple-touch-icon-152x152-precomposed.png?v=3 HTTP/1.1" 200 2522 "-" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:58.0) Gecko/20100101 Firefox/58.0"
x.x.x.x - - [03/Jun/2018:10:21:45] "GET /images/apple-touch-icon-72x72-precomposed.png?v=3 HTTP/1.1" 200 3990 "-" "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:58.0) Gecko/20100101 Firefox/58.0"

(This seems to happen with regular favicon.ico/.png files as well)

Obviously the bulk of your browsing will use the switched agent, but since you've exposed your real one upon your first visit to the site it doesn't make a difference. What's more, you've made it clear you are using a fake agent, which in a way exposes more information about you than if you were just browsing with your unaltered user agent.