Need to reload
Start score in any flat key.
add notes so that a naturalized note is the last note in a bar EG: Key Bb, note E natural.
Add anoher e natural as the first note in the nex bar.
Try to tie over bar line.
Tie does not appear.
Reload and tie appears.
Comments
I'm not able to reproduce this (the tie appears right away for me). Maybe you can provide more details (for example the method you use for adding notes, the method you use for adding a tie, what version of MuseScore are you using, are there other factors that affect this, etc.)
Tested using r.1833, Windows XP