Offsets in text frame are lost if combined with text frame margins

2.1 version 871c8ce/ Windows 10

1) Default "Untitled" score
2) Append or insert a text frame
3) Enter some text
4) Click on the frame border and in Inspector -> set the Left margin to 10.00mm
5) Select the text, and always in Inspector -> set the horizontal offset to 10.00sp
6) Save/Close/Reload

Result:the horizontal offset is now to 15.67 and will grow after each new save.
Test file at this step (so, first save): margin text.mscz

- Same behaviour with combination Top margin (eg 2.00mm) + Vertical offset to 2.00sp.
After save/close/reload, the vertical offset is now to 3.13sp: top margin text.mscz

- First reported in: https://musescore.org/en/node/223516
With some other considerations: https://musescore.org/en/node/223516#comment-736936