Bug report as requested when I reported it on the steam discussions. The booster nose cone on the right hand side has no fuel in it even though the slider is at maximum and it was placed using symmetry with the other booster which does have fuel.
It's easy to fix (just put the slider down and back up it will reset to having fuel) but here is the report anyway so hopefully you can figure out and prevent this from happening for the future.
https://steamcommunity.com/app/870200/discussions/0/1742230617622990524/
Dev Notes: This happens when the part is added while the 'Auto Resize Parts' option is disabled. Easy to reproduce. Just delete the nose cone, disable that option, and then add a new nose cone. One is correct, the other is showing incorrect mass. This appears to happen with all fuel tank parts.