I created a score this summer with 5.1.40, just opened it yesterday with 5.1.81.2225, and the location of the large time signatures has changed for no reason that I can pinpoint. As far as I know, I made no changes to the score, I just reopened the dormant file. Did Dorico change the implementation of how it places LTS? 
Screenshots and Dorico session attached. (Also another nudge to give us more granular control over where to place large time signatures—some clients and their house styles have strong opinions 
)
Update: It appears to be stemming from my use of top and bottom guide staves (top for video sync cues, bottom for a certain style of bar numbers). They’re a hack, but a hack that worked as I intended (and didn’t mess up LTS placement) in 5.1.40. Which leads me to nudge again for better implementation of these types of staves, they’re very common in some of the worlds I operate in.
Thanks!
5.1.40:
5.1.81.2225:
Big sigs.dorico (1.5 MB)