An 8va marking above the staff, followed by an extension line, tells you to play the affected notes an octave higher. When the line ends, you return to written pitch. If you identify C and D correctly but fail to track that endpoint, the note names can be right while the whole passage lands in the wrong register.
For example, a final written C5 within the line sounds as C6. If the next note, outside the line, is written D5, it must sound as D5, not continue conveniently to D6. Two neighbouring notes on paper appear to be a step apart, yet require a very different movement on the keyboard. Reading is not simply translating each notehead into a fixed position: it also means knowing which instructions are currently in force.
My practice suggestion is to understand the last note inside the line and the first outside it as one transition. Establish their actual registers, listen to the distance between them, then reconnect the surrounding phrases. The aim is not to manufacture a sudden leap, but to stop the eyes interpreting a new note under an expired instruction. The beginning of an octave line is conspicuous; its endpoint deserves just as much attention. Preparing only for the start is not enough.
ScoreFlow
To check register, you could use ScoreFlow, developed by Geoffrey Chen, to play back an imported MusicXML score and compare the sounding pitches. The file itself still needs proofreading: playback offers a reference for checking, not a guarantee of a correct edition.
https://handbook.musescore.org/notation/pitch/octave-lines
https://ultimatemusictheory.com/octave_sign/
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