tie and slur
There is general a missinterpretation of the so called tie.
If there is a slur between 2 notes of the same pitch the musician interprets this as tie. So the tie is a special kind of slur. In notation the tie can tendentially be more flat arc to underline a bit the different function of this special slur. For the musician however the tie is NOT recognized by the shape of the slur.
If a slur is between two notes of the same pitch and there is no articulation included it is to be read as a tie and the two notes are played as if it were one.
(Hope you can follow because my english is far from being perfect)
MuseScore as others has a special Element for ties as there is no articulation. Moreover it is - let me say - better style to draw it bit different than other slurs. For this element everyone obviously accepts that, if a tie is between two measures, that an accidential is also automatically transfered from one to another measure. So if there is a tie e.g. starting from an alterated tone at the end of a measure the note at the end of that slur in the next measure will get the same alteration.
As I said, we can't expect that musicians differenciate slurs and ties by its shape. So in consequence there are cases where a slur is between two notes and over a bar on the same line, but should be of different pitch. Then the slur could be interpreted as tie if an alteration is not reset.
Another case could be a portato notation (slur + articulation) over a bar for notes of the same pitch. This should be handled exactly like ties except for the articulation.
Unfortunately all I found in the internet about ties was not that clear to avoid a missunderstanding.
Sibelius at least automatically sets the correct accidentials also in special cases of slurs. If you are in doubt, please do not ask notation specialists without a good musical background.