If I open font A on GlyphMini and Font B in another window, if I try to select and paste all together the cyrillic letters on the cyrillic section of font B they looks messy and random. Same for other languages.
Check the UPM setting for those fonts in Font Info > Font. I think Roboto has 2048, if I remember correctly.
Use the Scale button next to the UPM field (do NOT just enter a new UPM value). Click the Scale button and then enter the value of the other font (e.g. 1000). Then copy the glyphs.