This has been bugging me for ages in several different fonts.
Here I want to sync the lsb and width of ra-myanmar with wa-myanmar. Here I haven’t synced it yet, it’s 10 units too wide:
When I hit cmd-ctrl-m, the width syncs but the lsb is now incorrect.
Clicking either of the ↻ update buttons syncs that value but unsyncs the other one.