Javascript attributes do not match HTTP headers when using TBB 9.0.2 on Linux

TBB 9.0.2 on Linux uses the UA

Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Firefox/68.0

for HTTP requests, but returns the UA

Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0

when requested by JavaScript. This can be seen when using the fingerprint viewer at amiunique.org