I copy a glyph and want to paste special the Width on another glyph, but All Masters is grayed out. Works in other software 
The “All Masters” in this case refers to the Add components option. Copying metrics to all masters wasn’t requested, yet. I’ll have a look.
Do you need to sync the metrics only for single glyphs, or maybe for the whole font?
in this case single. But for composites without metrics keys, or in a monospaced project that might have errors, paste special is fast.
Just trying to understand the use cases.
For monospace and for fonts where all masters need the same spacing, there are global custom parameter to control this.
I’ll see if I can add “all masters” for the metrics pasting.