Bug in display of tempo marking on https://musescore.com/ ?
My tempo marking as seen in Musescore on Windows 10: 
The same after upload to musescore.com: 
The crotchet symbol has been mangled.
I tried deleting and remaking it but same result every time.
Score is here https://musescore.com/user/72156/scores/3123301
Comments
I guess you used the "Reset custom style" button (the big quarter note tells it), I had the very same issue yesterday.
See https://musescore.org/en/node/307370#comment-1010415