I imported this Sibelius file into Dorico via MusicXML and my tremolos are displayed twice as fast (as 16th notes) in Dorico. They are played correctly as 8th notes though.
Sibelius:
Dorico:
When I eventually switch to Dorico, I will of course use ‘force duration’ to get back the original formatting on all of these. When I do that, they are correctly displayed as 8th note tremolo’s. (Is there a kind of mass editing feature for this by the way?) But I thought I share this here anyway because it may be an unknown bug. (As a sidenote: does it make sense to display a tie between tremolo notes in the first place? 
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Which version of Sibelius are you exporting from? I couldn’t make it happen from the latest version … BTW, there is a setting in Dorico prefs under MusicXML (Note durations) which emulates Force Duration on import 
The export was made from Sibelius 2019.1 into Dorico 2.2 (now I have Dorico 2.2.10). The strange thing is that they play correctly as 8ths 
Thank you for your remark on force duration on import, I will use that when I make my definitive import to switch to Dorico!
You risk being beaten to death by the percussionists here, if you say things like that (bad pun intended). See https://www.steinberg.net/forums/viewtopic.php?f=246&t=149339 for example.
Actually, I just discovered that the XML import option Note Durations MUST be active for the tremoli to import verbatim as well (!)