For a lot of reasons, I advise people to include at least 1 second of SMPTE bars at the beginning of every file, particularly one coming into Resolve (or any other color correction system). That way, you can check on scopes to make sure there's no shift in video level, black level, chroma, or hue.
These articles cover why color and levels can change with some video formats and some playback engines:
"Grading for Mixed Delivery: Cinema, Home, and Every Screen in Between" by Cullen Kelly
https://blog.frame.io/2019/10/14/gradin ... -delivery/"How to Deal with Levels: Full vs. Video" by Dan Swierenga
https://www.thepostprocess.com/2019/09/ ... l-vs-videoand
"A Deeper Look at Consistent Color with QuickTime Tags From Resolve To YouTube & Vimeo on Wide Gamut Apple Monitors"
by Dan Swierenga
https://www.thepostprocess.com/2020/07/ ... ktime-tagsand I think they cover the issues and the solutions very well. Understanding color management is also helpful:
"Color Management for Video Editors"
https://jonnyelwyn.co.uk/film-and-video ... o-editors/