eighth note swing

See:
https://musescore.org/en/handbook/swing

Note they would normally be straight by default. Only if the score was specifically created using the swing settings (eg, in Style / General, or by adding a Swing element from the Text palette) would it swing. My guess is a swing text was added and then perhaps edited to no longer say swing. If you need further assistance, please attach your score so we can check it out for ourselves.

Any system text can be made to turn on and off swing. You have apparently used system text somewhere and accidentally turned on swing. It's also possible it was turned on in Sytle->General->Score.